The 'I think' that must accompany everything

In the notoriously difficult Transcendental Deduction, Kant asks why the categories are entitled to apply to experience. His answer rests on a deceptively simple observation he calls the transcendental unity of apperception: the 'I think' must be able to accompany all my representations.

Here's the point. For a jumble of perceptions to be my experience of one connected world, they must be bound together in a single self-consciousness. A thought I couldn't own, or a perception that belonged to no unified 'I,' would be nothing to me at all. This unity of the self is not learned from experience — it's the condition that makes unified experience possible.

And that unifying act is the work of the categories. To combine scattered intuitions into 'one experience belonging to one subject' just is to synthesize them under concepts like substance and cause. So the categories necessarily apply to whatever can appear to us — because both the objects and the unified self are constituted by the same synthesizing activity.