From what I gather, MP denies the usual opposition between perception and imagination. According to MP, imagination doesn’t occur after one perceives something, but rather imagination is a key component of perception itself. In The Visible and the Invisible, he writes about a red dress whose red must be considered different depending on the context in which it appears or depending on the images it conjures in one’s mind. No one ever perceives red and only red; if one perceives red, it’s always a particular red, defined by the context and images it’s linked to in one’s mind. To imagine means to create, and to think of, contexts and mental images, which never fails to happen when one perceives something.
It’s all an objection to Sartre’s ideas about perception and imagination. According to S, there can be no confusion in one’s mind between what one perceives and what one imagines. (Consequence of his Cartesian idea that consciousness is what it appears to itself.) It doesn’t satisfy MP, since it cannot explain why one can value the products of one’s imagination—hallucinations, fantasies, etc.—as highly as reality itself. He ends up rejecting Sartre’s Cartesian assumption, and the clear distinction between perception and imagination with it.
From Phenomenology of Perception, Routledge, p. 399.:
”A woman patient declares that someone looked at her at the market, and that she felt the gaze fall upon her like a blow, but could not say whence it came. She cannot bring herself to say that in common property space there stood a flesh and blood person who turned his eyes towards her-and it is because of this refusal that the arguments that we can bring against her leave her completely unmoved. For her, it is not a matter of what happens in the objective world, but of what she encounters, what touches her or strikes her. The food refused by the victim of hallucinations is poisoned only for him, but to this extent, it is poisoned irrefutably. The hallucination is not a perception, but it has the value of reality, and it alone counts for the victim. The world has lost its expressive force,' and the hallucinatory system has usurped it. Although hallucination is not a perception, there is a hallucinatory deception, and this is what we shall never understand if we take hallucination to be an intellectual operation. However different it is from a perception, hallucination must be able to supplant it, and exist for the patient in a higher degree than his own perceptions. This can be so only so long as hallucination and per. ception are modalities of one single primordial function, through which we arrange round about us a setting of definite structure, through which we are enabled to place ourselves at one time fairly and squarely in the world, and at another marginally to it.”
(There aren't many instances of the word "imagination" in PoP, but I think this might count as a definition considering what MP writes in V&I.)
In PoP, "imagination" occurs ×17 and "imagine" ×25 (and mostly used in its trivial sense). Somewhat surprising! I'd say that both perception and imaginative supplementation of perception is active.
If the very nature of existence consists in being affected, then it's only apparent passivity, since it's just another way of saying that we actively structure the world by perceiving the objects in it, and by doing so we determine our place in it and among the objects in it.
