 Treatise III is also concerned with predication and God. But it differs sharply from the other treatises, in that it contains nothing specifically Christian. The question it addresses is how all substances are good in that they are, and yet are not substantial goods. Boethius takes it as a fundamental truth that all things tend to the good, and also that things are by nature like what they desire. Everything, therefore, is by nature good. But if so, then things must be good either by participation, or substantially (or ‘essentially’ as a modern philosopher would say). If they were merely good by participation, they would be good by accident, not by nature. But if they are good substantially, then their substance is goodness itself, and so nothing can be distinguished from the first good, God. In giving his answer, Boethius makes use of a set of axioms he states at the beginning of the piece, and undertakes a thought-experiment in which it is supposed per impossibile that God does not exist. The key to his solution lies in finding a principled way to distinguish between a thing a being F in that it exists, and a thing a being substantially F. For a to be substantially F means, Boethius’s discussion implies, that ‘a is not-F’ is inconceivable (we might say ‘logically impossible’). For a to be F in that it exists means just that ‘a is not-F’ is impossible (we might say ‘impossible given the way the world is set up’). Whereas it is inconceivable that God is not good, it is merely impossible that everything is not good.