"According to them, imagination is the striking of an imprint in the soul. On this point they were divided from the outset. Cleanthes took 'imprint' [literally] to mean a combination of ridges and indentations, such as result from the stamp of signet-rings in wax. Chrysippus however found such a notion absurd. To begin with, he says, thinking simultaneously of a triangular figure and a rectangular one would require the imaginative organ to take on different outlines at the same time, simultaneously assuming the shape of a triangle and a rectangle or a circle, which is absurd. Furthermore, when many imaginings arise in us at the same time, the soul would be seized by a multitude of outlines, which is less persuasive still. So he understood Zeno's use of 'imprint' to mean an 'alteration,' rendering the formula: 'Imagination is an alteration in the soul.' The absurdity is removed, if a single organ is to take on a multitude of alterations when many imaginings arise in us at the same time, just like the air: when many people are speaking at the same time it receives untold different blows at once, and sustains many alterations. The ruling element beset by elaborate imaginings will undergo something analogous to this."
"For when we call a man the composite of a soul and a body, or death the separation of soul from body, we are speaking in particular of the [soul's] ruling element. Likewise, whenever we make choices by virtue of their benefits to the soul, the body and its accessories, it is not the entire soul we are reflecting on but its ruling part, to which affections and benefits accrue. And so when Zeno says that 'Imagination is the striking of an imprint in the soul,' we are not to understand not the entire soul but a part of it, and to render the formula as 'Imagination is an alteration of the ruling element.' But even when it's put this way objections remain, for impulse, agreement and apprehension are alterations of the ruling part, and are not the same as imaginations. Whereas imagination is essentially a passive disposition of ours, the former are active faculties. So it's a bad definition, prescribed for too many different cases. It's like when someone defines 'man' by saying 'Man is a rational animal': the meaning of 'man' is unsoundly adumbrated, since a god is also alive and rational. Thus whoever deems imagination to be 'an alteration of the ruling element' goes wrong, since this no more describes it than any of the motions listed here.
"In answer to an objection like this, Stoics revert to 'implications,' saying it is necessary to understand that 'by a passive faculty' is additionally meant. For just as the definition of eros as 'the urge to create a close relationship' implies 'with youths in bloom' (even though this is not stated explicitly, given that no one feels eros for old men past their prime), they say that to define imagination as 'an alteration of the ruling element' implies an alteration resulting from a passive faculty, and not an active one. But not even by these means do their beliefs escape reproach."
Sextus Empiricus, Against Logicians I.227-31, 234-40