 Marmodoro’s concern about Aristotelian unity also indicates a difference between hylomorphism and the actual nature of substances, namely concerning the vagueness that attaches to most of our substance concepts. For most of our concepts, there are paradigm cases, but also marginal cases. This is an almost inevitable consequence of taking seriously the atomic constitution of objects, for there will be no very neat constraints on how they might combine. There are many theories about how one should react to vagueness, but one of the most common and intuitive is to say that it is a conceptual matter—reality itself cannot be vague. This tends against a realist and in favour of a conceptualist approach to our substance concepts: if our macro–substance concepts are prone to vagueness, and if vagueness is a sign that the concept in question is not to be taken in a realist fashion, then Locke would be right that what we normally regard as substances are just our ways of classifying things.