It has often been said that the knowability of substances became a problem in the early modern period, when anti-Aristotelians doubted that we could know anything more than the sensory qualities that are present to us. This article argues that the late scholastic Aristotelian Francisco Suárez was already aware of this sceptical problem. On his view, substances are really (and not just modally) distinct from the perceivable qualities, and therefore cannot be known through sense perception. The article first examines the (...) metaphysical theory that motivated him to defend this thesis. It then looks at the epistemological consequences he drew from it. Though he rejected direct knowledge of substances, he nevertheless conceded that knowledge can be obtained through a “discursive process”. The article explores this process, spelling out all the cognitive steps it involves. In particular, it analyses Suárez's explanation of how we produce special cognitive devices (the “intelligible species”) that enable us to represent substances. Finally, it assesses Suárez's solution to the knowability problem by comparing it to Locke's solution. It argues that metaphysical rationalism led him to posit substances: we need to accept them as active causes and bearers of qualities, although we have no direct access to them. (shrink)