A PRIORI and A POSTERIORI. The philosophical distinction expressed by these terms is to be explained by referring to the phraseology of Aristotle. According to him there may be a double starting-point in knowledge. When our individual progress in learning is chiefly considered, the things with which we are first and best acquainted may be termed earlier and prior; whereas the truths of a more general, primary, and fundamental character, to which we are subsequently led, have a later and posterior position. But if we lose sight of our personal interest in knowledge, then the priority may be more justly claimed by whatever is the cause or principle from which something else springs. In this sense the causal, original, and primary in the objec-tive world is by nature prior (Trporepov <j>vo-ei) ; whilst the secondary and derivative existence is posterior. Priority in the first acceptation is only relative to us, and for general purposes may be called accidental ; in the second acceptation, the priority is absolute, and without qualifica-tion. It is the second acceptation which Aristotle laid down as the properly philosophical one, and which regulates the usage of the phrases a priori and a posteriori by the schoolmen. In that sense of the term there can evidently be no a priori demonstration of first and fundamental principles. According to Aquinas, for example, there can be no a priori knowledge of God ; because He, as uncaused and uncreated, cannot be deduced from anything prior in causation to Himself, and can only be apprehended ration-ally by means of that which is consequent upon His action, viz., the creatures of the natural world. In other words, our knowledge of Him must be a posteriori. It is obvious that science in the highest sense must be a priori, if vere scire est per causas scire; i.e., the knowledge must spring from an insight into causes, which are the true primaries. By an extension of this usage an argument is said to follow an a priori path, when from the basis of some conception it proceeds to evolve by analysis all or some of the logical consequences; whereas, the mode of reasoning, which endeavours to gather into a single formula the various facts of observation, is described as a posteriori. The argument of Anselm, which, from the mere conception of God, pro-poses to deduce his existence, is an example of a priori reasoning. An a priori reasoner has to predict what is or will be, by considering what ought to be in accordance with certain presuppositions. He tries by argument to assign beforehand the place of a fact which may not yet have been discovered by observation. From the analysis of certain given conditions, or by constructing the total product from some given elements, he seeks to anticipate experience. Of course, if the original conception be bad or defective, the conclusions will be false or inadequate. Often too, what claims to be the mere deduction from a conception, is secretly and perhaps unconsciously supplemented by more efficient elements of proof. It is this circumstance, that imperfect knowledge is taken as the ground for further conclusions, which has brought a priori reasoning into dis- credit. Apart from these defects, however, this style of argumentation merely expresses the natural and blameless tendency of the mind to make every acquired truth a sort of lever and fulcrum from which to move the yet undis- covered and untried; and error is introduced only where there is a failure to correct this tendency by constant recurrence to the processes of verification. The a priori argument is based upon what was originally given through experience ; but before this experiential truth becomes a priori, it must lose its first and empirical character, and be invested with the attributes of universality and necessity. In this sense, which since the time of Kant has been com- monly given to the word, the a priori is the opposite of the empirical and contingent. Any truth which is rela- tively universal and necessary may in its own sphere form the basis of an a priori argument; and if there be anything which is absolutely universal and necessary throughout the whole range of knowledge, it will be in a supreme sense a priori. Such, according to Kant, is the self-contained and original faculty of mind, the forms and powers of the intellect and senses, as contradistinguished from the mate- rials presented by'the senses and elaborated by the under- standing. To ascertain the special constitution of this a priori region, thus marked out by the criteria of universality and necessity, to determine the features of thought when it is independent of and prior to all experience, was the theme of the Critique of the Pure Reason. The general or universal form and faculty of knowledge, thought in its native purity, constitutes the a priori element: whilst the particularising data of experience, drifting in from the unknowable thing-in-itself, make up the region of the a posteriori. An example of an a priori science, according to Kant, in this sense of the term, as not dependent upon experience, is seen in pure mathematics; and there may also, according to him, be a pure or a priori philosophy of nature; but there can be no pure or a priori doctrine of the ultimate ideas which regulate experience,—that is, metaphysics in the older sense is impossible. (W. W.)