Substance is something unknown, and is conceived of as possessing those qualities which are necessary to furnish us with simple ideas. But from the fact that substance is a product of our subjective thinking, it does not follow that it has no existence outside of ourselves. On the contrary, this is distinguished from all other complex ideas in the fact that this is an idea which has its archetype distinct from ourselves, and possesses objective reality, while other complex ideas are formed by the mind at pleasure, and have no reality corresponding to them external to the mind. We do not know what is the archetype of substance, and of the substance itself we are acquainted only with its attributes. From considering the conception of substance, Locke next passes over to the idea of _relation_. A relation arises when the understanding has connected two things with each other, in such a way, that in considering them it passes over from the one to the other. Every thing is capable of being brought by the understanding into relation, or what is the same thing, to be transformed into something relative. It is consequently impossible to enumerate the sum of every possible relation. Hence Locke treats only of some of the more weighty conceptions of relation, among others, that of identity and difference, but especially that of cause and effect. The idea of cause and effect arises when our understanding perceives that any thing whatsoever, be it substance or quality, begins to exist through the activity of another. So much concerning ideas. The combination of ideas among themselves gives the conception of knowing. Hence knowledge stands in the same relation to the simple and complex ideas as a proposition does to the letters, syllables and words which compose it. From this it follows that our knowledge does not pass beyond the compass of our ideas, and hence that it is bounded by experience.