[Essay IV xvii 14-19] Knowledge of relation requires only...

[Essay IV xvii 14-19] Knowledge of relation requires only that we are aware of non-identical connections among ideas, so (as we'll see in greater detail later) Locke supposed it possible wherever we have clear ideas, especially among the simple modes of number in mathematics and the mixed modes of human action in morality.

[Essay IV vii 6] But since general knowledge of relations can never be derived from experimental observation-upon which we depend entirely for our knowledge of material things-it follows that we can never have certain knowledge of relations among substances. [Essay IV vi 10, 16] What careful observation does provide is knowledge of the co-existence of a collection of qualities in a common subject.

But because we are ignorant of the real essences from which observable qualities presumably flow, our knowledge of co-existence is never demonstrable,