There are two elements in all knowledge: an _a posteriori_ element which comes from experience, and an _a priori_ element which comes from the thinking subject. The first is the _matter_ of knowledge; the second is the _form_. Separate, these two elements are unproductive. With the first alone we have but a reality not known; with the second alone we have but a knowing without reality. Their union renders them mutually fruitful by organising the data of experience into the necessary forms of thought. The principle of causation, _e.g._, is not in things; it is in the mind, and it is the mind which spontaneously connects all phenomena. Science, at bottom, consists in nothing but the causal connection of things. Where the chain breaks, positive knowledge ends. This clear sense of ignorance on points on which we really are ignorant is still a part of science and one of its principal forces, for it proves that it knows itself very well, and also knows the conditions apart from which it no longer exists. But, whether triumphant or held in check, positive science can neither renounce its task and method nor modify their nature. It can only seek to complete, or rather to lengthen, the chain of phenomena. The success of this ever-identical effort, an effort always in the same direction, is what is called its conquests and its progress. It follows that the irresistible tendency of science will be to extend over the whole of the phenomena the ever-tighter network of an invincible necessity. Determinism is its last word.