 Locke has a further argument up his sleeve: generality does not exist in nature. For the Aristotelian tradition, perception occurs when the mind receives the form (i.e., sensible species) without the matter of the substance. So, the human mind has access to forms (which are general) through perception. However, in the lead up to III.iii.15, Locke stresses that generality occurs only in words and ideas, not in nature (III.iii.1, 10, 13), and that we do not perceive substantial forms (II.xxiii.3; II.xxxi.6). Nominal essences provide the only universality we possess in natural philosophy (III.iii.9, 10, 11).