In recent years, Kant’s epistemology has received growing attention among scholars. A significant impetus for this field of research has been provided, in particular, by the pioneering work of several authors who have highlighted the complexity and epistemological significance of Kant’s thought, especially with regard to his theory of assent (holding-to-be-true, or Fürwahrhalten), which in some respects anticipates contemporary debates on belief and propositional attitudes (Mattey 1986; Stevenson 2003; Chignell 2007a; Chignell 2007b; Pasternack 2014; Höwing 2016). Other scholars have further developed this approach, focusing on various issues related to Kant’s epistemology of assent. Most have analyzed in greater detail questions concerning the notion of moral faith – such as its epistemic status and communicability (Pasternack 2011; Fonnesu 2015) – as well as the role of testimonial belief (Scholz 2001; Gelfert 2006; Shieber 2010). More recently, contributions have explored Kant’s non-evidentialism (Chance 2019; Gava 2019), fallibilism (Gava 2016; Chignell 2021), and doxastic voluntarism (Kohl 2015; Cohen 2024; Benzenberg 2024).

Less attention, however, has been devoted to another key concept in Kant’s epistemology – namely, certainty. Although this notion plays a significant role in the development and systematization of Kant’s theory of assent, most scholars have focused on specific aspects of his account of certainty – primarily its role and epistemic status in relation to moral certainty (Fonnesu 2011; Di Giulio 2024) – while leaving aside other central questions, particularly those concerning the very nature of certainty and its function in Kant’s taxonomy of holding-to-be-true.

In this paper, I aim to offer a broader perspective on Kant’s account of certainty. More specifically, I seek to clarify how, and to what extent, certainty can be interpreted as a feeling of the human soul in Kant’s philosophy. My claim is that certainty can be conceived as a feeling for Kant only if it is properly situated within his epistemology of assent. I will argue that Kant develops an epistemic account of certainty, in which certainty – broadly identified with the necessity of holding-to-be-true – can be understood in two distinct but closely related ways: as the quality of beliefs that attain the highest epistemic status and as the feeling of conviction involved in the act of truth-acceptance. To support this claim, I will relate Kant’s account to two major representatives of the German Enlightenment who significantly influenced his epistemology in general and his understanding of certainty in particular: Georg Friedrich Meier and Christian August Crusius.

I will begin in Section 2 by examining Meier’s logical writings, showing how he develops what I define as an objectivist account of certainty. According to this view, certainty is understood as a perfection of cognition, consisting in the clear and accurate representation of truth in the mind. In Section 3, I show how Crusius opposes Meier by formulating what I call a subjectivist account of certainty. For Crusius, certainty is essentially a feeling of inner compulsion arising from the psychological impossibility of conceiving the opposite as true. I then address Kant’s position in Section 4, explaining how his epistemic account of certainty gradually emerges through a critical confrontation – initiated in the pre-critical period – with both Meier and Crusius. More specifically, I argue that Kant develops his mature account of certainty in the critical period to overcome the two main limitations of his predecessors’ views: the evidentialism inherent in Meier’s objectivism and the radical psychologism characteristic of Crusius’s subjectivism. Finally, in Section 5, I analyze in greater detail the dual meaning of certainty in Kant’s epistemic account and show how this can help to resolve certain interpretative issues in Kant’s epistemology, most of which concern the sufficiency of holding-to-be-true.

The notion of certainty plays a central role in Meier’s logical reflections and is one of the most extensively discussed topics in the Vernunftlehre and the related Auszug.1 The importance of certainty in Meier’s account becomes clear from the definition he provides:

Apart from truth – which Meier considers the fundamental quality of knowledge3 – certainty is the most important feature of learned cognition, as it represents a “double perfection” (Meier 1752a, § 45) that requires both truth and clarity (Klarheit) to be effective. According to Meier, a cognition is true when it is in “agreement with its object” (Meier 1752b, § 99) and is therefore correct (richtig) in that it “not only appears to be a cognition, but also is one in fact” (Meier 1752b, § 93). This agreement between cognition and object can be established by either internal or external characteristics (Kennzeichen/criteria) of truth – namely, by the principles of non- contradiction and sufficient reason4 or by experience.5 Because it faithfully represents its object on the basis of sufficient characteristics of truth, a true cognition must also be both conscious and clear. Indeed, the same marks (Merkmale/notae) or characteristics that ground the truth of our knowledge are “those in the cognition or in the matters, which, when cognized, [are] the reason[s] why we are conscious of them” (Meier 1752b, § 115). When a cognition “contains as many marks as are required for consciousness” (Meier 1752b, § 124), it attains clarity.

To be defined as certain, a cognition must meet two requirements: (1) it must contain only true representations of things, and therefore must not be incorrect or misleading, and (2) its truth must be consciously cognized by the mind of the one who judges. While (1) is a general requirement for any perfect cognition, (2) specifically defines the proper nature of certainty, which emerges as a subjective perfection of human knowledge – distinct from, but closely related to, objective truth:

Unlike truth, which maintains a strong connection with the object of knowledge, certainty is a subjective perfection that concerns the cognitive activity of the mind. More specifically, certainty is that quality of knowledge that arises when the cognitive subject clearly recognizes the truth of a proposition and correctly accepts it on the basis of sufficient marks. In this sense, certainty is related to the subject’s doxastic attitude, which reflects his willingness to give or deny assent6 rather than suspending it:

On Meier’s account, certainty is the expression of our epistemic activity and depends on how we access truth. When our cognition of truth is sufficient, we achieve a more or less perfect form of certainty7 and can express a solid and legitimate assent. Conversely, when our cognition of truth is insufficient – that is, when our knowledge remains uncertain and thus not yet perfect8 – the validity of our assumptions depends on the rigor of our rational inquiry:

In the former case, we achieve a more or less probable cognition which, though imperfect, is still an approximation to certainty (Meier 1752b, § 171); in the latter, “we act rashly (praecipitantia)” (Meier 1752b, § 168) – that is, we judge without sufficiently investigating the marks of truth, and our cognition devolves into mere prejudice.

From this brief reconstruction, it becomes clear that feelings play only a marginal role in Meier’s account of certainty. For Meier, certainty is not a sentiment but a property of cognition, whose degree of perfection depends on the correctness and solidity of our rational investigation of truth. This does not mean, however, that Meier entirely disregards the subjective consequences arising from the possession of certainty. Like any other perfection of learned cognition, certainty generates a feeling of pleasure in us, as pleasure (Vergnügen/ voluptas) is, by definition, “the intuitive cognition of perfection” (Meier 1752b, § 237). Thus construed, pleasure is both the result of our learned cognition and the subjective drive motivating its attainment:

Yet while certainty undoubtedly arouses feelings, it is not, according to Meier, a feeling in itself. Similar observations apply to the notion of conviction (Überzeugung/convictio), which, while not explicitly defined by Meier as a feeling, more accurately captures the subjective consequences of certainty:

According to Meier, conviction is the state of the mind that arises from the acquisition of extensive certainty (ausführliche Gewissheit/certitudo completa) – a form of certainty based on such a complete cognition of the truth that the subject’s mind is rendered entirely free from fear of the contrary: “Through extensive certainty the mind is always assured in such a way that it is raised above all rational fear of the contrary” (Meier 1752b, § 159).

Despite these general references to the subjective states associated with certainty, Meier explicitly distinguishes conviction from both certainty and assent. Although closely interconnected, certainty, assent, and conviction remain distinct components of the cognitive process: certainty is a quality of cognition itself; assent is the cognitive act through which the subject relates to truth; and conviction is the subjective state that results from achieving full certainty. Meier’s primary aim in discussing conviction in his logic is not to analyze the subjective implications or emotional aspects of certainty but to clarify the validity of certain cognition – specifically, to distinguish those cognitions that are truly certain from those that are only apparently certain. This distinction relies on the opposition between conviction and persuasion (Überredung/persuasio):

While conviction is the result of a full and solid cognition of truth, persuasion is that deceptive state of the mind that derives from a faulty exercise of our rational faculties:

Thus construed, persuasion, as opposed to conviction, should be understood primarily in a methodological rather than merely psychological sense, as an imperfection of cognition that must be avoided through the application of logical rules in order to achieve the certainty proper to scientific knowledge:

In conclusion, Meier’s reference to the subjective states associated with certainty (persuasion and conviction), as well as to the transition from the former to the latter, serves the broader purpose of the doctrine of reason, whose primary goal “is the perfection of a learned cognition” (Meier 1752b, § 3). This goal can be achieved not only by defining the nature of certainty – which represents the pinnacle of this perfection – but also by eliminating all imperfections that hinder its attainment: uncertainty, as opposed to certainty, and illusory certainty (persuasion), as opposed to authentic certainty (conviction).

The notion of certainty also plays a central role for Christian August Crusius – so much so that it appears in the title of his major work on logic: Path to Certainty and Reliability in Human Cognition (Weg zur Gewißheit und Zuverläßigkeit der menschlichen Erkenntniß). Crusius’s account of certainty, however, is completely different from Meier’s, as can be seen from the definition he offers at the beginning of Chapter X, which concludes the first part of the work:

Unlike Meier, who understands certainty primarily as a quality of cognition, Crusius describes it as a psychological condition of the human mind characterized by two complementary feelings: the aforementioned sense of calm and pleasure derived from the absence of fear of the contrary, and a sense of inner compulsion (Zwang) that forces the subject to accept something as true without hesitation (“We find ourselves compelled [gezwungen] under certain circumstances to take some things to be certain,” Crusius 1747, § 421).

Crusius’s reference to the necessity of certainty based on the exclusion of the contrary would seem to parallel Meier and other rationalists, who associated certainty with forms of coerced assent arising from the recognition of the falsehood of the contrary. Crusius, however, explicitly rejects this parallel and challenges “those who understand certainty as a quality [Beschaffenheit] of the thing or of cognition, by virtue of which its opposite is not possible” (Crusius 1747, § 420). According to Crusius, the impossibility of the contrary – which grounds the certainty of cognition – cannot be limited to the logical impossibility derived from the principle of non-contradiction. For Crusius, this is merely a formal principle of thought that cannot ensure the effective extension of human knowledge and can at most guarantee only geometrical certainty (geometrische Gewissheit) – that is, a specific form of certainty pertaining to mathematical propositions and grounded solely in formal criteria of truth (Crusius 1747, § 423). The greater part of certainty experienced in human cognition is what Crusius terms disciplinary certainty (disciplinalische Gewissheit) – a form of inner compulsion arising from the fact that one “perceives [wahrnimmt] an impossibility in thinking of a thing otherwise, in separating or combining certain concepts” (Crusius 1747, § 423).

The impossibility of the contrary, which produces a state of certainty in the human mind, is thus the result of a psychological rather than a logical necessity, deriving from the nature of the understanding itself. According to Crusius, the understanding is properly the “capacity to think, i.e., to have certain concepts, to separate and to combine them” (Crusius 1747, § 255). This capacity has limits, as certain concepts and mental operations are conceivable while others are not. For Crusius, the intrinsic quality of our thinking power (wesentliche Beschaffenheit unserer Denkungskraft) is thus the fundamental touchstone of rational inquiry, which he summarizes in one supreme principle of knowledge:

This principle is both the primary ground of truth and the foundation of all certainty in human knowledge:

Crusius’s psychological account of certainty has important implications. The first is that Crusius is more radical than Meier in identifying certainty with the subjective mechanisms of truth-assumption, which he describes through the notion of assent or holding-to-be-true (Vorwahrhalten):

As we have seen, Meier also relates certainty to assent: certainty consists in the clear representation of truth, which translates into firm and justified assent based on the recognition of sufficient marks. Nevertheless, certainty and assent play different roles in Meier’s logic: the former is a quality of cognition, while the latter is its epistemic counterpart, reflecting the subject’s relation to truth based on a more or less adequate investigation of marks. By contrast, Crusius, in describing both certainty and assent as psychological states (Zustände) of the mind, tends to overlap them. For him, certainty is essentially the feeling of constriction that we experience when we are free from fear of the contrary, which in turn derives from the psychological impossibility of thinking the opposite as true. The same absence of fear that compels us to accept the truth of a proposition also instills a sense of calm in the mind, generating a feeling of satisfaction that restrains the will9 from pursuing further investigation:

The second implication of Crusius’s psychologism is the identification of certainty with conviction. As mentioned above, Meier distinguishes conviction from certainty: for him, conviction is the subjective manifestation of extensive certainty and, more specifically, the expression of the methodological legitimacy of true certainty, in contrast to the false and apparent certainty that gives rise to persuasion. Crusius, for his part, uses the terms Gewissheit and Überzeugung interchangeably to describe the same phenomenon – namely, the feeling of inner constriction based on the supreme principles of knowledge: “The three aforementioned principles of reason should serve only as the ground of conviction [Grund der Überzeugung] in our understanding” (Crusius 1747, § 265). In Crusius’s logic, conviction loses its methodological meaning – which in Meier’s logic derives largely from its opposition to persuasion – and becomes synonymous with certainty and, indirectly, with firm assent. If conviction, like certainty, consists in the subjective constraint that compels an individual to accept a proposition as true and restrains his will from further investigation, the parallelism between certainty, conviction, and assent becomes evident: “The inner sensation [Empfindung] of a compulsion in the understanding to accept a proposition as true is called conviction” (Crusius 1747, § 447).

Meier and Crusius develop opposite accounts of certainty. Meier supports what I term an objectivist account: although he regards certainty as a subjective perfection of cognition – in the sense that it is the representation of truth in the human mind – certainty remains strongly dependent on objective truth. For Meier, certainty is a fundamental property that any cognition must possess in order to become perfect and arises when our knowledge reflects objective truth as faithfully, accurately, and vividly as possible, on the basis of sufficient marks. By contrast, Crusius provides what I call a subjectivist account of certainty. According to him, certainty is primarily a subjective phenomenon of the mind, resulting from the conformity of our cognition with the highest principle of human knowledge, interpreted in psychological terms. Thus construed, certainty consists in the psychological impossibility of thinking the opposite as true, which gives rise to two complementary feelings: the feeling produced by the absence of fear of the contrary and the feeling of inner compulsion to accept the object of cognition as true.

Meier’s and Crusius’s accounts serve as the principal points of reference for Kant’s early inquiry into certainty. In the pre-critical period, Kant appears to be more sympathetic to Meier’s objectivist account. In the Blomberg Logic, for instance, he explicitly adopts the general definition of certainty that Meier provides in § 29 of his Auszug, albeit with some reservations: “The author explains certainty here ‘as the consciousness of the truth of a cognition’. For the time being this explanation is good enough, and sufficient” (V-Lo/Blomberg, AA 24:57). A similar observation appears in an early reflection dating back to the mid-1750s and related to the same § 29. Here, Kant agrees with Meier in defining certainty as a form of distinct cognition of truth: “Certainty is cognized truth [erkannte Wahrheit]; it has degrees, and these depend on the distinction [Deutlichkeit] of cognition” (Refl. 1767, AA 16:107). Following Meier’s example, Kant, while insisting on the subjective nature of certainty, does not sever the connection between certainty and truth,10 which he generally understands as the “agreement [convenientia] of an idea or cognition with the object” (Refl. 1765, AA 16:107).11 According to Kant, certainty consists in a more or less solid cognition of truth that, in accordance with Meier, must be based on sufficient grounds of knowledge: “Certainty […] arises from nothing but the relation of equality between the grounds of cognition [Erkenntniß-gründen] that I have and the whole sufficient ground itself” (V-Lo/Blomberg, AA 24:144). The objective validity of cognition, as well as its degree of certainty, derives from its conformity to the principle of sufficient reason, which Kant conceives of as a cognitive principle:12

A cognition is thus certain for Kant if it accurately represents the truth of a proposition or, in subjective terms, if one has sufficient grounds of knowledge to hold it to be true. In this respect, certainty “is to be regarded as a unity, and as a complete whole, and thus is the measure of all the rest of our holding-to-be-true, and of each and every one of its degrees” (V-Lo/Blomberg, AA 24:144). The relationship between the grounds of cognition one possesses and the whole of sufficient reason determines the degree of certainty of one’s cognition, which may be more or less probable or merely doubtful: “if there is even one more degree of truth on the side of the insufficient ground than there is on the side of the opposite, then the cognition is […] probable”; otherwise, “[w]hen the moments of holding-to-be-true constitute exactly half of the sufficient ground, then a cognition is […] doubtful” (V-Lo/Blomberg, AA 24:144).13

Despite his agreement with Meier, the pre-critical Kant is aware of the limitations of a purely objectivist account of certainty. If certainty consists solely in the clear representation of truth, then we can regard ourselves as legitimately certain only with respect to those cognitions that rest on a sufficient number of objective grounds. Put differently, we are justified in holding only those beliefs for which we can provide a more or less adequate amount of evidence. In the terms of contemporary epistemology, then, an objectivist account of certainty implies an evidentialist account of belief. This is the position that Meier maintains in his logical writings. As we have seen, he makes certainty dependent on the characteristics of truth one is able to cognize – that is, on the grounds of proof available to one’s cognition:

This means that the attainment of full certainty depends on our ability to cognize a sufficient number of grounds of truth, which can be derived from only three possible sources – first-hand experience, second- hand experience (i.e., testimony), and reason:

The fact that there are only three sources of cognition limits the number of positive doxastic attitudes – namely, the forms of truth-acceptance – that a subject can legitimately manifest: knowledge (Wissen), opinion (Meinen), and Belief (Glaube).14 Knowledge is a form of firm assent based on a rational cognition of truth, and it is proper to science, which consists in a learned cognition made extensively certain by rational investigation (Meier 1752b, § 185). Opinion is an uncertain doxastic attitude, in which the subject holds an uncertain proposition to be true while being aware of its uncertainty: “An opinion (opinio) is any uncertain cognition, insofar as we accept it, and at the same time cognize that it is not certain” (Meier 1752b, § 181). Finally, Belief (Glauben) consists in holding something to be true on the basis of second-hand experience – that is, the testimony of a witness deemed sufficiently credible:

According to Meier, then, the only legitimate kinds of assent are those that are objectively justified, namely, those in which one can exhibit a sufficient amount of evidence, ideally balancing all three main sources of cognition,15 to ground one’s belief and the corresponding degree of certainty. Meier regards the process of increasing the certainty of learned cognition as a process of enhancing the overall perfection of our knowledge, progressively making our cognition more solid, truth-accurate, and convincing by carefully investigating the cognitive grounds of assent through the rules of logic:

This account is questioned by Kant in the pre-critical period. In particular, Kant explicitly challenges Meier’s understanding of Belief (Glaube) as assent based on testimony. In the main text of the Bauch Logic, he comments on § 206 of Meier’s Auszug as follows: “Our Author refers to the holding-to-be-true [Fürwahrhalten] for the sake of testimony as [B]elief [Glaube]. However, we believe many things, [...] even when there is no testimony” (Kant 1998, p. 150). In the Blomberg Logic, Kant draws a crucial distinction:

According to Kant, then, Glaube is not merely synonymous with testimonial Belief – that is, a form of assent based on specific kinds of objective grounds, such as the credibility of the witness and the degree of verisimilitude of the reported facts. For Kant, Glaube has a broader and more nuanced meaning, directly related to the sphere of morality and pragmatic agency: “To hold something to be true immediately, to the extent necessary for action, is called believing. Conversely, to hold something to be true on the basis of someone else’s statement is called believing someone” (V-Lo/Philippi, AA 24:448, my emph.). Already in the pre-critical period, Kant introduces a significant distinction – absent in Meier – between logical, objective grounds for assent, which underlie “objective” forms of holding-to-be-true such as knowledge, opinion, and testimonial Belief, and practical, subjective grounds, which form the basis of a different kind of Belief that does not involve logical necessity:

By acknowledging the legitimacy of holding certain cognitions to be true even in the absence of objective grounds, Kant begins to outline a non-evidential account of belief that allows for non-epistemic – namely, practical – forms of justification.16 This rejection of Meier’s evidentialism also compels Kant to rethink Meier’s objectivist account of certainty. If we are justified in giving our assent on the basis of non-evidential – that is, non-objective – grounds, then we must acknowledge that one can legitimately be certain of something even without knowing the objective characteristics of its truth. In this respect, an objectivist account that defines certainty as a reflection of truth in the mind based on sufficient marks appears overly restrictive. In several passages of his logical corpus, Kant describes certainty in a way that emphasizes the subjective aspects of its attainment rather than its objective implications in terms of truth-depiction. Especially in the Philippi Logic, certainty is represented as a subjective phenomenon, related to a state of inner conviction and absence of doubt, rather than a representation of truth derived from a clear cognition of grounds:

This approach to certainty is less dependent on an objectivist understanding of certainty as something tied to truth and brings Kant closer to the subjectivist account developed by Crusius, who identifies certainty with an inner compulsion and the absence of fear of the contrary. In an interesting passage of the Philippi Logic, Kant offers a description of certainty that closely resembles Crusius’s view not only in content – through the identification of certainty with the necessity of assent and the state of conviction – but also in terminology, notably in the use of Vorwahrhalten (a term absent in Meier) and in the reference to an innerer Zwang:

This Crusius-influenced understanding of certainty leads the pre-critical Kant to define it explicitly as a “feeling [Gefühl] of the human soul” (V-Lo/Philippi, AA 24:421), directly related to the necessity of holding-to- be-true and the force of subjective conviction, which in turn is described as a feeling in the 1764 Prize Essay (UD, AA 2:295). Crusius’s influence on Kant is evident in the fact that both authors emphasize not only the psychological implications of certainty but also the connection between subjectivism and non-evidentialism. In fact, Crusius also develops a non-evidential account of belief that partially anticipates Kant’s position. For Crusius, too, Glaube is not merely a matter of testimonial Belief; rather, in its narrower sense, it refers to “what one must merely believe in contrast to what one perceives and what one clearly and indubitably recognizes through the path of demonstration” (Crusius 1747, § 477). This kind of Belief generates a certainty grounded in moral necessity, according to which the believer is forced to hold a probable proposition as true due to either a prudential obligation (Verbindlichkeit der Klugheit), related to private ends, or a proper legal obligation (gesetzliche Verbindlichkeit), tied to the moral ends imposed by God’s will (Crusius 1747, § 413).17

Although Kant is sympathetic to Crusius’s non-evidentialism, he remains reluctant to endorse a fully subjectivist account of certainty. His reservations about Crusius’s subjectivism and psychologism are evident in the same 1764 Prize Essay, where he explicitly refers to a feeling of conviction (Gefühl der Überzeugung) in relation to the certainty of fundamental metaphysical truths. More specifically, in this text Kant questions Crusius’s idea that certainty should be reduced to a mere psychological necessity, arising from a principle of human cognition that cannot ground any truth and manifesting itself as a state of conviction that lacks objective justification:

From the pre-critical period onward, then, Kant’s challenge is to develop an account of certainty that is liberal enough to go beyond Meier’s objectivism and evidentialism yet rigorous enough to avoid collapsing into Crusius’s psychologism, which falls short of justifying the legitimacy and objective validity of our truth- assumptions.

A definitive solution to this issue comes with the critical turn. During the 1780s, Kant develops his mature account of certainty, which involves a reinterpretation of key themes from the German logical debate of the 18th century. Kant’s strategy is to provide a new general definition of certainty, making it neither dependent on the notion of truth, as in Meier’s account, nor reducible to psychological phenomena, as in Crusius’s account. His solution is to emphasize the epistemic implications of certainty – that is, the doxastic attitude of the knowing subject toward truth, rather than the cognition of truth itself. In doing so, Kant further develops an idea that Meier only briefly touches upon and that Crusius formulates in psychological terms through the notion of Vorwahrhalten. In Kant’s mature account, certainty and uncertainty are not forms of complete or incomplete cognition of truth, but rather expressions of a necessary or unnecessary kind of holding-to-be- true:

Kant’s reference to the necessity of holding-to-be-true could be seen as a concession to Crusius, who likewise defines certainty as the feeling of inner compulsion that forces us to accept something as true. However, the necessity of holding-to-be-true described by Crusius merely expresses the psychological impossibility of thinking the opposite as true, in accordance with a highest principle of cognition that is itself conceived in psychological terms. As we have seen, Kant does not deny that certainty has psychological effects on the mind. Nonetheless, for Kant, understanding the necessity of certainty exclusively as a feeling of compulsion amounts to interpreting as a psychological necessity what is, above all, an epistemic necessity – that is, the necessity of a cognitive act based on specific grounds of assent. This is what Kant means in the Canon of Pure Reason, where he defines holding-to-be-true as “an occurrence [Begebenheit] in our understanding that may rest on objective grounds [Gründen], but that also requires subjective causes [Ursachen] in the mind of him who judges” (KrV, A 820/B 848).

The development of what I term an epistemic account of certainty during the critical period allows Kant to overcome the limitations of both Meier’s objectivism and Crusius’s subjectivism. By focusing on the subject’s epistemic activity, Kant offers a definition of certainty that no longer relies on a strong reference to objective truth. In Kant’s epistemic account, certainty is understood as a “logical perfection of cognition as to modality” (Log, AA 9:65). This means that, for the critical Kant, certainty remains a perfection, as it does for Meier. This perfection, however, consists not in a clear and accurate representation of truth in the mind but rather in the proper justification of our beliefs – namely, in the necessary character of the assent one is willing to give to a propositional content based on sufficient grounds. The degree of certainty, as well as the kind of assent one is able to express, then depends directly on the nature and sufficiency of the reasons that underlie one’s holding-to-be-true:

This less demanding account of certainty allows Kant to qualify as certain all cognitions that imply a strong epistemic commitment from the subject. These cognitions may constitute proper knowledge – a necessary holding-to-be-true supported by a sufficient number of objective grounds of truth and thus producing a theoretical certainty valid for everyone19 – but they may also include simple Belief (Glaube), which lacks objective justification yet generates moral certainty due to its subjective sufficiency. In this respect, Kant’s epistemic account represents a step forward from Meier’s objectivism and evidentialism. At the same time, Kant’s focus on the epistemic phenomena related to certainty accounts for all the subjective implications of certainty that Crusius emphasizes, such as the feelings of inner compulsion, absence of fear, and conviction. However, these feelings are no longer the foundation of certainty and truth in the human mind but rather the result of the subject’s epistemic activity, which finds its own justification in either epistemic or non-epistemic grounds. In this way, Kant’s approach preserves Crusius’s understanding of certainty as a subjective feeling of the human soul without falling into mere psychologism.

On Kant’s approach, certainty – generally conceived as the necessity of the subject’s epistemic commitment to a given proposition – can be further articulated in two distinct yet closely interconnected ways. Certainty in the first sense (C₁) pertains directly to the epistemic nature of our beliefs – more specifically, to the fact that they possess the highest possible epistemic status. Certainty in the second sense (C₂) can be understood as the subjective state that accompanies our epistemic activity. Thus conceived, C₂ is properly a “feeling of the human soul” (V-Lo/Philippi, AA 24:421) and coincides with the feeling of conviction expressing the solidity of our holding-to-be-true. In this respect, certainty (C₁) and conviction (C₂) are not separate concepts but rather two ways of describing the same epistemic phenomenon from different perspectives: the subject’s ability to express a belief with the highest epistemic status and the feeling of conviction that accompanies her act of truth-acceptance.20

The distinction between C1 and C2 helps to clarify a controversial interpretive issue in Kant scholarship – namely, how to understand the sufficiency of holding-to-be-true, and of subjective sufficiency in particular. As we have seen, in several passages of his logical corpus Kant describes the three kinds of holding-to-be- true (knowledge, Belief, and opinion) based on their objectivity/subjectivity and sufficiency/insufficiency. More specifically, knowledge is an objectively and subjectively sufficient holding-to-be-true, Belief an objectively insufficient and subjectively sufficient holding-to-be-true, and opinion an objectively and subjectively insufficient holding-to-be-true.21 Objective sufficiency seems easier to grasp, as Kant associates it with the epistemic legitimacy of holding-to-be-true, which derives from the possession of objectively and thus universally valid22 grounds of assent. Subjective sufficiency, by contrast, remains more controversial. Kant uses the expression subjektive Zulänglichkeit – along with the related adjectives subjektiv zulänglich and subjektiv hinreichend – to denote the cogency of assent that characterizes both knowledge and Belief. In the case of knowledge, however, this subjective sufficiency follows directly from the existence of objective reasons: she who knows can provide adequate objective justification for her knowledge. As a result, her holding-to-be-true is both objectively necessary and subjectively firm. Belief, on the other hand, relies solely on subjective sufficiency to avoid being reduced to mere opinion in the absence of objective grounds.

This distinction has led some scholars to argue that Kant recognized two kinds of subjective sufficiency – one pertaining to knowledge and one to Belief. Chignell, for instance, interprets the former as the subject’s ability to identify the objective grounds of assent upon reflection, whereas he considers the latter a form of justification based on non-epistemic merit (Chignell 2007a, pp. 329, 333–335; cf. also Chignell 2007b, pp. 44ff., 50ff.). Stevenson adopts a similar view, asserting that Kant distinguishes “between two different conceptions or standards of justification” (Stevenson 2003, p. 84): a theoretical one, which pertains to knowledge, and a practical one, which pertains to Belief. This interpretation appears to be supported by those passages in which Kant claims that the grounds for Belief are not speculative and provide sufficient justification only within a practical context. In the Critique of Pure Reason, for example, he writes: “Only in a practical relation, however, can taking something that is theoretically insufficient to be true be called believing” (KrV, A 823/B 851). Similarly in the Busolt Logic: “Holding something to be true on the basis of subjective reasons that are sufficient from a practical point of view is Belief” (V-Lo/Busolt, AA 24:647).

Chignell’s and Stevenson’s idea of the duality of subjective sufficiency is essentially based on the assumption that different grounds of assent (epistemic and non-epistemic) give rise to different forms of subjective sufficiency (theoretical and practical). However, as Pasternack (2014, pp. 44-45n.) correctly points out, this assumption “introduc[es] an unnecessary and textually unwarranted distinction onto Kant.” To avoid this distinction, Pasternack proposes a unified account of subjective sufficiency, which he simply understands as the psychological state of “firmness” (Festigkeit) in holding a proposition to be true (Pasternack 2014, pp. 42-44). Although I am sympathetic to Pasternack’s unified account of subjective sufficiency, I do not agree with his equation of subjective sufficiency with the notion of Festigkeit. This term is used by Kant in the Orientation essay to describe the immutability of moral faith (moralischer Glaube) (WDO, AA 8:141), but it rarely appears in Kant’s writings and lacks the theoretical significance needed to fully characterize subjective sufficiency.

A more consistent way of describing objective and subjective sufficiency is to identify the former with C₁ and the latter with C₂. Kant’s willingness to describe both kinds of sufficiency in these terms emerges from a series of Reflexionen from the late 1770s. In Refl. 2465, for instance, he writes: “Certainty: objectively necessary holding-to-be-true, which, when it is also subjectively sufficient, is conviction” (AA 16:382). Similarly, in Refl. 2596: “The necessity of holding-true is, when it is subjective, conviction; when it is objective, certainty” (AA 16:434). This idea is later reaffirmed in a previously quoted passage from the first Critique: “Subjective sufficiency is called conviction (for myself), objective sufficiency, certainty (for everyone)” (KrV, A 822/B 850). As these passages clearly show, the distinction between the objective and subjective sufficiency of holding- to-be-true, as well as the unitary character of subjective sufficiency, naturally emerges from Kant’s broader account of certainty. If certainty is neither the mere reflection of objective truth in the mind (as in Meier) nor a simple psychological occurrence (as in Crusius), but rather an epistemic phenomenon tied to the act of holding-to-be-true, then it becomes easier to see that certainty can take two distinct forms: on the one hand, as the subject’s ability to express a necessary holding-to-be-true that meets the highest standards of epistemic justification (C₁, corresponding to the objective sufficiency proper to knowledge); on the other hand, as the feeling of conviction, which is the subjective awareness of the necessity of assent (C₂, corresponding to the subjective sufficiency proper to both knowledge and Belief). This framework offers a general account of the sufficiency (objective = C₁ or subjective = C₂) and insufficiency (objective = lack of C₁ or subjective = lack of C₂) of holding-to-be-true, without implying any dualism within subjective sufficiency or endorsing a psychologistic interpretation of it based on the generic notion of Festigkeit.23

One might raise two objections to my account. The first concerns the fact that in several passages of his logical writings, Kant mentions two forms of conviction: theoretical conviction, which pertains to knowledge, and practical conviction, which pertains to Belief. This distinction might seem to support the idea of the dual nature of subjective sufficiency. However, this objection can be overcome by recognizing that, in distinguishing between theoretical and practical conviction, Kant is actually referring to the grounds of assent rather than the conviction that follows from them. A conviction is termed “practical” solely because it is based on non-epistemic grounds. Holding something to be true may or may not be epistemically justified, but in either case, its subjective sufficiency – that is, the feeling of conviction it produces in the subject – remains the same. Although closely related, the sphere of justification, which concerns the number and nature of the grounds for assent, must be clearly distinguished from the sphere of certainty, which pertains to the epistemic completeness of the act of truth-acceptance (C₁) and the subjective awareness of its necessity (C₂). On this basis, Kant can affirm that the feeling of conviction in both knowledge and Belief is not only identical in nature but also equal in strength: “But there is also a [B]elief in connection with which I cannot alter my holding-to-be-true at all; […] this is a practical [B]elief. Hence it is fully as strong as conviction and as the greatest apodeictic certainty” (V-Lo/Wiener, AA 24:851).24 Similarly, this is why, for Kant, conviction (C₂) applies to both knowledge and Belief, as both possess subjective sufficiency, whereas it is entirely absent from opinion. Someone who is convinced gives her assent based on legitimate grounds, regardless of whether those grounds are sufficient to justify a holding-to-be-true from an objective point of view. This explains Kant’s claim, in the Vienna Logic, that “[o]pining is not yet conviction, for otherwise it would have to be at least subjectively sufficient, for me in the condition of mind in which I find myself” (V-Lo/Wiener, AA 24:850). From this perspective, subjective sufficiency corresponds to the conviction that arises when something is held to be true, whether it is objectively sufficient (knowledge) or objectively insufficient (Belief). Conversely, subjective insufficiency indicates the lack of conviction that characterizes mere opinion.25

A second objection might be raised at this point: if objective insufficiency is understood as a lack of C₁– that is, as a form of epistemic uncertainty resulting from insufficient objective grounds – how can Belief, which is a subjectively sufficient but objectively insufficient holding-to-be-true, give rise to moral certainty? To address this issue, we must take into account the peculiar nature of this kind of certainty and, more importantly, the epistemic processes that determine its formation. In knowledge, certainty primarily consists in the necessity of holding-to-be-true based on objective grounds, which in turn gives rise to subjective certainty – that is, the feeling of conviction. In other words, C₁ produces C₂ in knowledge. Conversely, the certainty we express in Belief is exclusively C₂, whose legitimacy stems from a non-epistemic justification based on practical – that is, moral – grounds. This is what determines, in Kant’s view, not only the equal epistemic dignity of Belief in relation to knowledge but also its independence from the latter:

Moral certainty essentially consists in this “firmest conviction”: it represents the fullest and most solid form of C₂that one can legitimately hold on moral grounds, even in the absence of objective theoretical justification.

Due to these peculiarities, moral certainty is the expression of a merely assertoric Belief – that is, a complete but not objectively necessary holding-to-be-true, which remains private and thus incommunicable:26

The identification of moral certainty with C₂ is thus consistent with the two key features that Kant ascribes to moral faith: its non-speculative nature and its incommunicability. Whoever has faith – specifically in the existence of God, the immortality of the soul, and freedom of the will – cannot claim to have knowledge of these things, that is, a theoretical holding-to-be-true that can be shared with others. This is not only because the grounds of her assent are merely practical, but also because the certainty she experiences is only a subjective feeling (C₁), which, although the highest possible, is not supported by any objective certainty (C₁) based on speculative grounds. Since moral certainty corresponds to C₂, it shares the characteristics of any other feeling. The first is that “feelings can never produce a cognition” (V-Lo/Dohna, AA 24:730), meaning they are non-cognitive mental states that cannot “be incorporated in a cognition” (Cohen 2020, p. 4). The second is that feelings “are not as universally communicable as intuitions and concepts” (V-Lo/Wiener, AA 24:811; cf. also V-Lo/Dohna, AA 24:706; V-Lo/Philippi, AA 24:428).

Kant’s understanding of certainty, as it gradually emerges from his engagement with Meier and Crusius – from the pre-critical period to its final systematization after 1781 – is complex and multifaceted. It takes into account different perspectives: the subject’s epistemic commitment to the object of cognition, the nature of the grounds of assent, and the subjective states that accompany the different forms of holding-to-be-true. In doing so, Kant develops an epistemic theory of certainty in which certainty itself is conceived as a dual epistemic phenomenon, encompassing both the necessity and epistemic completeness of holding-to-be- true (C₁) and the feeling of conviction involved in this act (C₂).

This dual nature of certainty is what allows Kant to overcome the two major limitations of his predecessors’ accounts: (1) the evidentialism implied in Meier’s objectivism and (2) the psychologism inherent in Crusius’s subjectivism. Kant resolves the first limitation in two main steps. First, he weakens Meier’s understanding of certainty as a clear representation of truth by redefining it as a manifestation of the subject’s doxastic attitude. Second, he acknowledges the legitimacy of forms of holding-to-be-true – such as moral faith – where the subject’s certainty consists not in her recognition of objective marks of truth but rather in the strength of her feeling of conviction, based on moral grounds. At the same time, Kant ensures that his non- evidentialist account of belief does not collapse into a merely psychologistic interpretation of certainty, as it does in Crusius’s subjectivism. This is possible because certainty as a feeling (C₂), though conceived as a mental state, is not simply a psychological occurrence for Kant but rather the expression of the subjective sufficiency of holding-to-be-true. This means that C₂ cannot be reduced, as Crusius does, to a mental necessity tied to a psychological principle of human cognition. Rather, it conveys the strength and stability of the subject’s epistemic activity, which may derive either from the epistemic necessity of truth-assumption based on objective grounds (C₁), as in knowledge, or from non-epistemic grounds of assent, as in Belief.27