=2.= One of the great difficulties, throughout this controversy, is the extremely variable use to which these words, as well as the word _empirical_, are put by different authors. To Kant, who was nothing of a psychologist, _à priori_ and _subjective_ were almost interchangeable terms[1]; in modern usage there is, on the whole, a tendency to confine the word _subjective_ to Psychology, leaving _à priori_ to do duty for Epistemology. If we accept this differentiation, we may set up, corresponding to the problems of these two sciences, the following provisional definitions: _à priori_ applies to any piece of knowledge which, though perhaps elicited by experience, is _logically_ presupposed in experience: _subjective_ applies to any mental state whose immediate cause lies, not in the external world, but within the limits of the subject. The latter definition, of course, is framed exclusively for Psychology: from the point of view of physical Science all mental states are subjective. But for a Science whose matter, strictly speaking, is _only_ mental states, we require, if we are to use the word to any purpose, some differentia among mental states, as a mark of a more special subjectivity on the part of those to which this term is applied. Now the only mental states whose immediate causes lie in the external world are _sensations_. A pure sensation is, of course, an impossible abstraction--we are never wholly passive under the action of an external stimulus--but for the purposes of Psychology the abstraction is a useful one. Whatever, then, is not sensation, we shall, in Psychology, call subjective. It is in sensation alone that we are directly affected by the external world, and only here does it give us direct information about itself. =3.= Let us now consider the epistemological question, as to the sort of knowledge which can be called _à priori_. Here we have nothing to do--in the first instance, at any rate--with the cause or genesis of a piece of knowledge; we accept knowledge as a datum to be analysed and classified. Such analysis will reveal a formal and a material element in knowledge. The formal element will consist of postulates which are required to make knowledge possible at all, and of all that can be deduced from these postulates; the material element, on the other hand, will consist of all that comes to fill in the form given by the formal postulates--all that is contingent or dependent on experience, all that might have been otherwise without rendering knowledge impossible. We shall then call the formal element _à priori_, the material element empirical.