The Crusian Epistemology and Metaphysics

Kant’s criticisms of Crusius are overall, comprehensive and fruitful, and they almost touch every aspect of the latter’s theoretical philosophy. In spite of this, we cannot find any text that can

88 be paralleled in its entirety and systematicity to those on Plato; it is true that Kant’s Inquiry Concerning the Distinctness of the Principles of Natural Theology and Morality (1764)¹¹⁹ includes a most detailed and informative discussion of Crusius in the pre-critical period. However, Kant makes a quite different point on Crusius in his critical period; and the point he makes in Prolegomena (1783) and in the Critique (1787) also differs in important respects. Therefore, in the following, I will reconstruct Kant’s interpretations and criticisms of Crusius from a wide range of Kant’s texts.¹²⁰

Crusius maintains that there are three highest principles of reason concerning the distinguishing features of things and non-things. The first principle is (1) the principle of contradiction that nothing can be and not be at the same time, which is widely recognized for its validity. In contrast to the orthodoxical Wolffians, Crusius makes the crucial move of restricting the scope of the validity of the principle of contradiction. Crusius makes the contention that not all that cannot be thought can be explained in terms of the principle of contradiction, and thus he puts the non-logical unthinkability on a par with the logical unthinkability. In addition to the principle of contradiction, Crusius formulates two novel non-logical supreme principles: (A) the principle of the inseparable: what cannot be thought apart from each other also cannot exist apart from each other, and (B) the principle of the uncombinable: what cannot be thought with and next to each other also cannot exist with and next to each other.¹²¹

Crusius is not content with formulating the principle of inseparability and that of uncombinability; rather, he further explores the foundation of all these three principles. According to Crusius, the three previous principles are only parts of the highest distinguishing feature of the truth “that what cannot be thought as such is not possible or actual, and that, by contrast, what can be thought is possible”¹²². In effect, Crusius claims that epistemic conceivability and metaphysical possibility are co-extensive.

Although in Crusius’ mind the three highest principles are concerned with the existence of things, they are apt to be reformulated as concerned with the truth of propositions. Crusius draws a distinction between the formal and material principles in metaphysics. The principle of non-contradiction is the supreme principle of all formal principles, and the principle of inseparability and that of uncombinability are supreme principles of all material principles. In other words, the three highest principles could be construed as the grounds of the truths of these propositions. The

119 Abbreviated as Inquiry thereafter.

120 Again, what matters is not what Crusius says, but how Kant conceives Crusius. The following reconstruction is primarily based on Kant’s reception of Crusius in his works. This outlining of Crusian philosophy draws on the translation of Crusius by Watkins 2009.

121 See Watkins 2009, 140.

122 Watkins 2009, 140-141.

89 principle of non-contradiction is the highest principle that grounds all formal principles. The principle of inseparability and that of uncombinability are the two highest principles that ground all material principles.

Note that for Crusius the distinction is drawn within metaphysics, for which Crusius so writes:

“[t]here are two kinds of truths. Some are contingent, […] However, others are necessary[…].

Now, metaphysics ought to treat of necessary truths. [. . .]”¹²³. It is clear that for Crusius metaphysics is assumed to be concerned with necessary truths. By acknowledging the material metaphysical principle Crusius is committed to the view that there are necessary truths which are unexplainable by appeal to the principle of non-contradiction.

The status of the highest principle of understanding is ambivalent. In most cases, Crusius takes the highest principle of understanding as metaphysical. In other cases, Crusius also accords this highest principle with epistemic function, and he refers to it as the criterion of truth, by means of which we can distinguish truth from error. When it functions as an epistemic criterion of truth, it is reformulated by Kant as “what cannot be thought as other than true is true” (AA 2:295). In the following I will call it the C-criterion of truth (named after Crusius):

For any p, p is true if and only if p cannot be thought other than true.

So we have to reformulate the Crusian philosophy in Kant’s terminology such that we are able to be in a position to compare them. In my view, the Crusian formal/material distinction is co-extensive with the Kantian analytic/synthetic distinction. The formal principles of metaphysics rest on the principle of non-contradiction. Consequently, the material principles can be defined negatively as those that do not rest on the principle of non-contradiction.¹²⁴

The Crusian distinction between material and formal principles does not merely anticipate Kant, and it is even taken over by Kant. The Reflections dated to the middle and late 1760s lends evidence to this view. Kant adopts the terminology of material and formal principle in his unpublished notes in a typical Crusian way. More importantly, he uses metaphysical principles and

123 Watkins 2009, 137-138.

124 On a simplistic interpretation, these two distinctions are even synonymous. However, the complexity of Kant’s definition of analyticity should be underrated. Traditionally, it is the non-contradiction characterization, rather than containment characterization, lies at the center of Kant’s notion of analyticity. Recently, Anderson argues that the containment account of the analyticity is fundamental. For Kant’s threefold definition of analyticity see Anderson 2015.

90 synthetic ones interchangeably. In the Reflection 3922 dated to 1769 Kant writes: “[m]aterial principles seem to be: whatever happens, must have a ground. Every successive series has a beginning.” (AA 17:346) And in one following paragraph, Kant continues: “[a]nother synthetic principle is: whatever thinks is only a simple subject.” (AA 17:347)

In the R3923 Kant further links the analytic/synthetic distinction to the formal and material aspects of human cognition: “[s]ome principles are analytic and concern the formal aspect of distinctness in our cognition. Some are synthetic and concern the material aspect, in which case they are the arithmetical, geometrical, and chronological principles.” (AA 17:348)¹²⁵

In Kant’s terminology, to say that all truths in metaphysics are necessary is just to say that all propositions are a priori. By acknowledging the actuality of material principle in metaphysics, it is safe to conclude that Crusius is in effect committed to both the actuality thesis and the synthetic apriority thesis and thus to their conjunction that the synthetic a priori propositions are actual.

Since the explanation of the analytic propositions is as unproblematic for Cusius as for others, in the following I will reconstruct Crusius’ account for the possibility of the synthetic a priori propositions:

(4.1) The synthetic a priori propositions are actual.

(4.2) The ground of the synthetic a priori propositions is the C-criterion, namely, the nature of understanding. (C-criterion of truth)

(4.3) The C-criterion as the contingent mental structure is implanted by God. (Divine Reliabilism)

For (4.1): Compared to the Platonic argument, it is less awkward to reformulate the Crusian argument in Kant’s terminology. Crusius identifies a new kind of a priori knowledge which cannot be explained by the principle of non-contradiction. Kant considers this as a crucial progress; in his pre-critical work Inquiry, Kant writes: “Crusius is also right to criticize other schools of philosophy for ignoring these material principles and adhering merely to formal principles. For on their basis alone it really is not possible to prove anything at all.” (AA 2:295)

125 The difference between the two distinctions should be called into attention. For Kant analytic propositions are not truths in metaphysics, but instantiations of rules in logic.

91 The existence of the synthetic a priori propositions underpins the anti-empiricist view that not all knowledge is derived from experience. This point is less significant, for the actuality of the a priori propositions is commonplace in German rationalist tradition. In fact, the coextension between necessity, analyticity, and apriority lies at the heart of the German rationalist tradition. In a more significant sense, however, this move can be seen as a deviation from the traditional rationalist view by arguing that not all a priori knowledge draws on conceptual analysis.

For (4.2): Crusius does not only acknowledge the existence of the synthetic a priori propositions, and he is also committed to the assumption of the ground of truth that every truth must have a ground. Therefore, there must be a supreme principle of truth for the synthetic a priori, as the principle of contradiction for the analytic a priori propositions. Even if the synthetic a priori propositions are indemonstrable, initially it does not imply that there is no criterion for us to tell the true from the false. Crusius introduces the C-criterion as the criterion of the truth of the synthetic a priori propositions: for any proposition p, p is true if and only if p cannot be thought other than true. According to Crusius, the C-criterion of truth that “what cannot be thought as other than true is true” is identified with “the essence of our understanding”¹²⁶. The unthinkability or inconceivability is nothing other than the innate principle, or the contingent structure of human mind that is endowed to us along with our existence.

For (4.3): Kant refers to the C-criterion of truth as innate or implanted. If the C-criterion of truth is not an utterly contingent fact, then there must be an implanter of it. Not surprisingly, Crusius identifies this implanter as God. Since the C-criterion of truth is a general rule, this appeal to God revives Leibnizian pre-established harmony. The system of pre-established harmony is essentially a common cause model. On this model, Crusius claims that thoughts and things outside of thoughts co-vary without genuinely causing each other.

Kant agrees with Crusius that there are synthetic a priori propositions. However, Kant disagrees with the Crusian explanation of them, and he mounts a number of harsh criticisms of (4.2) and (4.3). The following two sections are devoted to examining them respectively. In this course, Kant reveals most of his fundamental assumptions, which turn out to have important consequence for making intelligible the motivations of Kant’s criticisms.