First, it must be possible in principle to arrange and organise the chaos of our many individual sensory images by tracing the connections that hold among them. This Kant called the synthetic unity of the sensory manifold.
Second, it must be possible in principle for a single subject to perform this organisation by discovering the connections among perceived images. This is satisfied by what Kant called the transcendental unity of apperception.
Experiential knowledge is thinkable only if there is some regularity in what is known and there is some knower in whom that regularity can be represented. Since we do actually have knowledge of the world as we experience it, Kant held, both of these conditions must in fact obtain.