Kant’s manner of formulating his fundamental problem--How are synthetic _a priori_ judgments possible?--may well seem to the modern reader to imply an unduly scholastic and extremely rationalistic method of approach. Kant’s reasons for adopting it have, unfortunately, been largely obscured, owing to the mistaken interpretation which has usually been given to certain of his personal utterances. They have been supposed to prove that the immediate occasion of the above formula was Hume’s discussion of the problem of causality in the _Enquiry into the Human Understanding_. Kant, it is argued, could not have been acquainted with Hume’s earlier and more elaborate _Treatise on Human Nature_, of which there was then no translation; and his references to Hume must therefore concern only the later work. Vaihinger has done valuable service in disputing this reading of Kant’s autobiographical statements. Kant does not himself make direct mention of the _Enquiry_, and the passages in the _Critique_ and in the _Prolegomena_[9] in which Hume’s teaching is under consideration seem rather to point to the wider argument of the _Treatise_. This is a matter of no small importance; for if Vaihinger’s view can be established, it will enable us to appreciate, in a manner otherwise impossible, how Kant should have come to regard the problem of _a priori synthesis_ as being the most pressing question in the entire field of speculative philosophy. The essential difference between the _Treatise_ and the _Enquiry_, from the standpoint of their bearing upon Critical issues, lies in the wider scope and more radical character of the earlier work. The _Enquiry_ discusses the problem of causality only in the form in which it emerges in _particular_ causal judgments, _i.e._ as to our grounds for asserting that this or that effect is due to this or that cause. In the _Treatise_, Hume raises the broader question as to our right to postulate that events must always be causally determined. In other words, he there questions the validity of the _universal_ causal principle, that whatever begins to exist must have a cause of existence; and he does so on the explicit ground that it demands as necessary the connecting of two concepts, that of an event and that of an antecedent cause, between which _no connection of any kind_ can be detected by the mind. The principle, that is to say, is not self-evident; it is synthetic. The concept of an event and the concept of a cause are quite separate and distinct ideas. Events can be conceived without our requiring to think antecedent events upon which they are dependent. Nor is the principle capable of demonstration. For if it be objected that in questioning its validity we are committing ourselves to the impossible assertion that events arise out of nothing, such argument is only applicable if the principle be previously granted. If events do not require a cause, it is as little necessary to seek their source in a