APRIL-FOOLS' DAY or ALL-FOOLS' DAY, the name given to the 1st of April in allusion to the custom of playing prac tical jokes on friends on that day, or sending them on fools' errands. The origin of this custom has been much disputed; it is in some way a relic of those once universal festivities held at the vernal equinox, which, beginning on old New Year's Day, March 25, ended on April I. In India, at the feast of Huli, the last day of which is March 31, the chief amusement is the befooling of people by sending them on fruitless and foolish errands. Though April 1 appears to have been anciently observed in Great Britain as a general festival, it was apparently not until the beginning of the 18th century that the making of April-fools was a common cus tom. In Scotland the custom was known as "hunting the gowk," i.e., the cuckoo, and April-fools were "April-gowks," the cuckoo being there, as it is in most lands, a term of contempt. In France the person befooled is known as Poisson d'avril.
A PRIORI AND A POSTERIORI. The literal meaning of these terms is prior and subsequent respectively; and the reference is usually to the relation of knowledge to experience. Generally speaking any knowledge or component of knowledge that is prior to experience of the facts in question is described as a priori, whereas any knowledge or cognitive constituent that is derived from experience of the facts concerned is said to be a posteriori. Both these terms have many different shades of meaning, the chief of which can be best appreciated by follow ing their historical course.
The Aristotelian meaning of the terms was intimately con nected with Aristotelian philosophy generally. According to this anything real is composed of a universal essence (or form) and matter. By nature the universal essence is prior to the mat ter. Hence any reasoning from the general nature (or essence) of things is argument from that which is by nature prior (a priori), whereas any knowledge derived from experience of the materialized instances of that essence is obtained from that which is by nature subsequent (a posteriori). The Aristotelian use of these terms has only survived in the much modified form accord ing to which any knowledge derived from general principles, or from the general nature of the objects contemplated, is said to be a priori, whereas knowledge derived from observation is de scribed as a posteriori. In this modified sense, a priori and a posteriori are almost equivalent to deductive and inductive re spectively.
The Scholastic meaning of the terms, as used by Albert of Saxony in the 14th century, was restricted to the purely tem poral distinction between them, and abstracted from the Aris totelian metaphysics of form and matter. As causes precede their effects, arguments from causes to effects were called a priori, those from effects to causes were called a posteriori.
The Rationalistic use of the terms was different again. Briefly whatever is derived from concepts, ideas, or reason as such is called a priori; whatever is learned from experience is called a posteriori. This usage was partly suggested already by Plato, and encouraged by Descartes and Herbert of Cherbury, but was brought into vogue mainly by Leibniz. Its chief importance con sists in the fact that it paved the way for the new uses to which Kant put these terms.
The Transcendental use of the terms, as introduced by Kant, ignores the temporal distinction and emphasizes the logical differ ence between them. What Kant distinguishes by means of them is not two kinds of knowledge but two moments or factors in all knowledge. In all human knowledge, according to Kant, there is something which is not derived from experience, something in fact without which human experience would be impossible— this factor he calls the a priori element in knowledge. Such a priori elements are the forms of space and time, the categories of substance, cause, etc. These are inherent in the nature of the mind as such, and are not derived from experience. On the other hand there is in all knowledge a something, a manifold something, supplied by experience or through experience—this factor, this raw material as it were, is the a posteriori element which is transformed into an orderly experience with the help of the a priori forms and categories. The a priori elements, as con ceived by Kant, though subjective in the sense that they are sup plied by the percipient mind and not by the manifold of sensation (the external raw material of experience), are objective in the sense that they are not peculiar to any individual mind but are characteristic of, and valid for, all experience. Again, the a priori elements are not things of which the mind is explicitly con scious; as a rule they only manifest themselves in the way in which they shape experience.
The Evolutionary use of the terms, as we find it in Herbert Spencer, G. H. Lewes and others, attempts to mediate between the transcendental and the genetic interpretations of the factors in question. What seems a priori to the individual may neverthe less have been slowly acquired from experience in the long his tory of the human race. Jf so the difference between the a priori and the a posteriori is not absolute—what is a posteriori to one generation may become a priori for some future generation.
(A. Wo.)