[Aristotle DA 428a24: ‘So it is clear that imagination (phantasia ) would be neither opinion accompanied by sense perception nor produced by sense perception, nor a combination (sumploke) of opinion and sense perception’.] Plato in the Sophist [264A] and Philebus [39Bff.] posits that imagination consists in a mixture (mixis) of opinion and sense perception, and with these words Aristotle seems to be making an objection to this position. But in truth he show; how one must understand mixture by his refutation of the notions put forward. For it is not the case that imagination has its being in this kind of activity of both, with sense perception coming first and opinion following on (this notion he indicated with ‘nor produced by sense perception’), nor with both happening simultaneously, nor with them coinciding (suntrekhein) but each one contributing its own activity in its simple form, not mixed (amikton ) with the other (one must think he means this when he says ‘neither opinion accompanied by sense-perception’), or with them coinciding and being combined with each other and no longer distinct. Rather there is one common activity resulting from the two, not by merger (sunkrisis) with some destructive change in each, as in the case of honeyed wine (for it is impossible to posit this in the case of immaterial entities), but with them remaining as they are and unmixed (eilikrines), but with all of each as it were penetrating the whole of the other. It is not, of course, just any kind of mixture of sense perception and opinion that is imagination. 
[212,12] [‘Imagination is not these] partly for this reason’, he says, meaning the things that have been mentioned, all the things that are peculiar to each of sense perception and opinion and distinguish appearance from them. As, for example, that in dream images there is neither potential nor actual sense perception, and if sense perception is of things which are present, imagination happens when sense objects are not present, and if opinion is not within our control and happens with assent to something as true and through persuasion and reasoning, while imagination is within our control and without belief or reason, imagination would not consist in the activity of both of those, either coming together or one coming first and the other following, both for these reasons and because of the argument which he now uses.
[212,21] So what mixture of them does Plato mean? The one which consists in the middle components being simple and themselves, but which is called a ‘combination’ (sunthesis) and a ‘mixture’ from the extremes because the middle terms have something in common (koindnia) with both the extremes. This is how Aristotle says the colours between come from white and black, and the Timaeus [35A] says that the soul comes from the substance that is undivided and the one that is divided in bodies, solely because of the participation of the middle terms in the extremes by virtue of their simplicity. For appearance too is in the middle between sense perception and opinion.