First, Plato argued that we see not by light entering our eyes (as we now know is the case) but by light proceeding out of our eyes (Timaeus 45b). For Plato, seeing is an active, not a passive, process. Since knowledge was considered to be a kind of seeing, Plato also construed knowing as acting on something rather than being acted upon (Sophist 248-49). I’ve discovered that this mistaken view of seeing and knowing is picked up and defended by a host of Hellenistic philosophers.
Second, several Neoplatonistic philosophers (Iamblichus, Proclus and Ammonius) used this theory of eyesight and knowing to explain how the gods can foreknow future free actions. They argued that the nature of divine knowledge is determined not by what is known but by the nature of the knower. Since they assumed the gods were absolutely unchanging, they concluded that the gods knew things in an absolutely unchanging manner, despite the fact that the reality the gods know is in fact perpetually changing. This allowed them to affirm that the future partly consisted of indefinite (aoristos) truths (viz. open possibilities) while nevertheless insisting that the gods knew the future in an exhaustively definite, unchanging way.
The view is, I’m convinced, completely incoherent. But one can understand how these philosophers arrived at it in light of their mistaken assumptions about seeing and knowing as wholly active processes. What the gods see when they look at the future conforms to the unchanging nature of the gods rather than the changing nature of the future they see. Through the influence of Augustine and especially Boethius (who explicitly espoused the ancient view of seeing and knowing and repeated some of the Neoplatonic arguments), this way of “reconciling” foreknowledge and free will quickly established itself as the dominant view in the Christian tradition.