Abstract: My aim is to reconstruct Kant's argument for the principle of the synthetic unity of apperception. I reconstruct Kant's argument in stages, first showing why thinking should be conceived as an activity of synthesis (as opposed to attention), and then showing why the unity or coherence of a subject's representations should depend upon an a priori synthesis. The guiding thread of my account is Kant's conception of enlightenment: as I suggest, the philosophy of mind advanced in the Deduction belongs to an enlightenment epistemology. Kant's conception of enlightenment turns on the requirement that a subject be able to recognize herself as the source of her cognitions. The argument for the apperception principle is reconstructed under the guidance of this conception of the ideal of enlightenment.