The matter stands quite differently however when we come to consider the validity of the ideas which we ourselves produce (in the three ways noted above) ! They cannot of course be copies or impressions! We use them however as archetypes, or patterns, with which to make comparisons. In this case, therefore, we estimate objects from the viewpoint of their agreement or disagreement with the patterns of our own construction. But compound, relative and abstract ideas furnish no information whatever as to the real nature of things. It is in this sense that we use figures in mathematics, and moral ideals in ethics. The proofs of mathematics and moral philosophy are wholly independent of the existence of the things to which they refer. But such is not the case with the idea of substance, which is expressly intended to indicate an external object. The validity of ideas of this kind can only be established therefore on the basis of a complex of attributes given in experience, or if, as in the case of the idea of God, we are in position to offer a separate proof for its validity.
In agreement with Descartes, Locke distinguishes between intuition and demonstration. Intuition merely furnishes us knowledge of self and of the simplest relations between our ideas. The combination of a series of intuitions results in demonstration. These two kinds of knowledge alone are fully certain; sense experience is always only probable. Locke proves the existence of God by appealing to the principle of causality: The world must have a cause, and, since matter cannot produce spirit, the cause of the universe must be a spiritual being. He regards our knowledge of the causal principle itself as an intuition, i. e. as self-evident. At this point he agrees with the dogmatic systematizers. Hence he
