In his Topics Aristotle criticises an alleged definition of being as ‘capable of being acted on or acting on (to dynaton pathein e poiesai)’ (Top. 146a21-32, 148a18-21, cf. 139a4-8). There is an echo of passages where Plato described ‘power of acting on and being acted on (dynamis tou poiein kai paschein)’ as a ‘horos’ of being in the Sophist, though he wavered between this conjunctive formulation (Soph. 248c8) and disjunctive ones (247d8-e2, 248c4-5). A ‘horos’ can be interpreted as a definition or a criterion (G. E. R. Owen insisted on the former meaning, criticising Cornford's preference for the latter). In this respect, Aristotle - who takes it solely disjunctively - would definitely have been on Cornford’s side, judging from his remarks on some of the deficiencies in disjunctive definitions.
In the Sophist, the Eleatic stranger asked whether ‘knowing and being known are cases of acting on, or being acted on, or both? Is one of them acting on and the other being acted on? Or is neither a case of either?’ (248d4-7). But in due course Theaetetus responds that if the Friends of the Forms insist on the Forms'unchangeability, they must choose the last option, i. e. in the case of the Forms, neither knowing nor being known is either acting on or being acted on. But this choice implies, in exchange for defending their unchangeability, that the Forms cannot be known. Insofar as Forms are known, they are changed by being acted on, and this is a serious obstacle for the Friends of the Forms. So Aristotle finds the crux of an argument in the equation of being acted on with being changed. And this becomes his target to be resolved.
In due course Aristotle must have realized these exchanges left behind one option unscrutinized, i.e. the knower is acted on by the Forms which the knower knows. In the case of sense-perception, though it was in dialectical arguments refuting Protagoras' secret doctrine in the Theaetetus, Plato had already suggested that perceiving is not acting on its objects but rather being acted on by the objects (Theaet. 156a-157b). But later in the Timaeus, Plato still treated sight-perception as an acting on rather than being acted on because he believes that in ‘the visual stream, like makes contact with like and coalesces with it to make up a single homogeneous body aligned with the direction of the eyes’ (Tim. 45b-46c, cf. 67c-68d). When Aristotle explains the so-called doxographical method of collecting preceding views, he mentions the following remarks; ‘We see by admission of something into ourselves, not by an emission; for so it is, too, in the case of the other senses; for in hearing we admit something into ourselves; we do not emit; and we taste in the same way.’ (Top. 105b6-9). This suggests that he criticizes Plato's analysis of vision without mentioning him and even tries to change that view by an analogy with the other senses. We can acknowledge this as a dialectical procedure like ‘change of mind’ (metabibazein) described in E. E. 1216b30 and Top. 101a33.......