Today we wrestled with Locke’s idea of substance. A substance is a fundamental entity, one that can ground explanations, and one that can support qualities (or accidents). Atoms are substances, and maybe – Locke seems to say – macro level things like our own bodies are too. Locke thinks we can have ideas of substances, but our ideas do not reveal them as they are in the mind independent world. That is because we construct our complex ideas of substances out of the simple ideas we gain in perception – and these ideas (1) include all sorts of ideas of secondary qualities, which are nothing like real substances, (2) are missing the crucial idea of support, which would explain what makes a substance able to keep all these qualities together! Instead of getting at real things, we group together ideas that seem similar, ideas of things as they seem to us. This is a problem for our ideas of physical substance, but also for our ideas of mental substances – i.e. minds, or souls.
Perhaps, Locke says, if we only had microscopical eyes, we would see atoms, which are substances, as they are. We don’t have such eyes, and so Locke worries that he faces a problem of perceptual evil: how can we believe that (a) God wants us to understand things through perception, (b) God is all powerful, and yet (c) God produced us with eyes that only see large things which produce in us inaccurate ideas of substances? Locke argues in reply that microscopical eyes would not be useful, and so that they represent a tradeoff between the accuracy and usefulness, and God understandably chose to give us eyes that are useful. That is Locke’s theodicy about perception.