For Aristotle, the object as perceived and the subject as perceiving can only be understood with respect to the activity of perception itself and the unity it brings about between the perceived object and perceiving subject. This is a complex unity that requires further analysis and refinement. This unity is the ground of Aristotle’s “realism” with regard to perception. However, if this unity is dissolved into an external, synthetic connection between two dissimilar things only one of which we have access to, then it is at best problematic and perhaps impossible for us to discover anything about the other. This is the source and origin of modern epistemological skepticism.
Bowler, Michael, "A Quarrel Between the Ancients and the Moderns: Aristotle's Realism and Modern Skepticism" (2004). The Society for Ancient Greek Philosophy Newsletter. 441.