Kant’s chief purpose in the _Critique of Pure Reason_ was to vindicate the certainty of the physical knowledge of a part as being compatible with total ignorance of the whole. The older metaphysics was engaged in the attempt to supply the whole, to sketch it out in order to give certainty to the part that is within the reach of science. The older metaphysics said to science: You have in hand the conditioned, but remember the conditioned depends on the unconditioned. Unless, therefore, you round out what you possess, with the help of the unconditioned, the certainty you seem to have within the field of the conditioned disappears. Again, science traces causes, and the older metaphysicians insisted that the whole chain of causes hangs in air unless it be attached to a first cause. Now Kant’s _Critique of Pure Reason_ really amounts _in nuce_ to this: you do not require the whole in order to explain the part. Link the partial phenomena together in a certain way, a way dependent on the joint action of the space and time intuitions and the categories, and you will gain the desired certainty. The certainty is in the linkage. We may add link to link of the chain of reality without troubling to consider by what piers it is supported or on what shore the piers rest—if indeed there be piers and shores at all. The bridge hangs over the River of Time and we can safely travel on it. How we get on to this bridge we do not know, and where we shall leave it we cannot know either. It is a mistake to speak of Kant as a rationalist pure and simple. When he expelled the older metaphysics he antagonized pure rationalism. The older metaphysics held that the mere existence of the conditioned proves the existence of the unconditioned, requires the unconditioned. In Kant’s answer to this lies the gist of his enterprise in philosophy: You are quite right, he says, that the _idea_ of the conditioned requires the idea of the unconditioned, logically, rationally. But observe well, nature is not just logical or rational. There is an irrational element in it, namely, extended manifold and temporal sequence. Juxtaposition and sequence are irrational, because, if I interpret him rightly, in the case of each the relation presented to the mind is that of parts outside each other—in the one case alongside, in the other before and after; while in the logical or rational relation the parts are implicit in the whole as in the case of the premises of a syllogism and the conclusion, the relation of a genus to the species, the universal to the particular.