Immanuel Kant - Imagination

Think more of "image-making" in regard to perception and cognition than today's employment of imagination as concerning fictional inventions. Imagination is the mediator between the understanding and sensibility through its temporal structuring of sensation. Going deeper than that is to indulge in the arduous and potentially painful task of sorting out the specific details of Kant's schematism. It's worth adding how imagination wound-up with synthesis:

Kelley L. Ross - The problems that must be sorted out with Kant are at the same time formidable. Most important is the confusion that results from Kant mixing together two entirely different theories in the Critique of Pure Reason. The first theory is that the fundamental activity of the mind, called "synthesis," is an activity of thought that applies certain concepts to a previously given perceptual datum from experience. It is upon this theory that the Critique of Pure Reason was planned with its fundamental division between the "Transcendental Aesthetic," about the conditions of perception (what Kant called empirical "intuition"), and the "Transcendental Logic," about the conditions of thought. Thus, Kant still says, as late as page 91 of the first edition ("A"), "But since intuition stands in no need whatsoever of the functions of thought, appearances would none the less present objects to our intuition" , without, that is, any need for mental synthesis.

However, right in the middle of his subsequent argument for why certain concepts would be necessary and known a priori with respect to experience, Kant realized that "synthesis" would have to produce, not just a structure of thought, but the entire structure of consciousness within which perception also occurs. Thus he says, "What is first given to us is appearance. When combined with consciousness, it is called perception". It is the structure of consciousness, through synthesis, that turns "appearances" into objects and perceptions, without which they would be nothing. Consequently Kant made synthesis a function of imagination rather than thought, as a bridge between thought and perception, though this creates its own confusions (it still depends on the forms of thought and is still treated in the Logic). This move occurred because Kant hit upon the idea that synthesis produced the unity that we actually find in "apperception," i.e. in the unity of consciousness -- everything I know, think, see, feel, remember, etc. belongs to my consciousness in one temporal stream of experience. Synthesis therefore brings things into consciousness, making it possible for us to subsequently recognize that our consciousness exists and that there are things in it. --Friesian entry on Kant