Kant calls the Principle of the Synthetic Unity of Apperception (PSUA) the “highest point” to which we “must affix all use of the understanding, even the whole of logic and, after it, transcendental philosophy.” In this article, I offer an original interpretation of this “supreme principle.” My argument is twofold. First, I argue that the common identification of this principle with the “I think” or even the form of the I think misses the basis on which this principle is capable of grounding Kant’s transcendental deduction. It must be understood as a purely formal, transcendental principle. Second, I argue that this highest principle must be understood (in part) as a purely formal principle of pure synthesis in order for Kant’s account of the mind to lay valid claim to such spontaneity (the freedom of theoretical cognition operative through synthesis) without invoking the very dogmatic idealism that he critiques. The reduction of this principle to the real I think, or even to the Transcendental Unity of Apperception (TUA), undermines the basic distinction on which Kant’s deduction depends. I pay particular attention to important, recent arguments from Longuenesse (2017) on the TUA, Williams (2017) on the Original Unity of Apperception, Allison (2004) and Pollok (2017) on the status of a priori principles, and McLear (2015) on the claim that unity necessarily presupposes synthesis.
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