 Buridan proceeds to identify four ways in which one can obtain one simple cognition from another directly, or without any inference [QP I.4, ed. Streijger &amp; Bakker:  36–37].[46]  The details need not detain us here, but suffice it to say that the “empiricist” side of Buridan's nominalism is always leavened by Aristotelian naturalism, which he felt was needed to combat skeptical arguments about scientific knowledge, and indeed, about the very existence of substances—arguments Ockham never had to confront.