The word has been used technically in philosophy with various shades of meaning. Thus it is used to translate the Platonic Idea (eidos), the permanent reality which makes a thing what it is, in contrast with the particulars which are finite and subject to change. Whether Plato understood these forms as actually existent apart from all the particular examples, or as being of the nature of immutable physical laws, is matter of discussion. For practical purposes Aristotle was the first to distinguish between matter (hyle) and form (eidos). To Aristotle matter is the undifferentiated primal element: it is rather that from which things develop than a thing in itself. The development of particular things from this germinal matter consists in differentiation, the acquiring of particular forms of which the knowable universe consists (cf. Causation for the Aristotelian formal cause). The perfection of the form of a thing is its entelechy in virtue of which it attains its fullest realization of function (De anima, ii. 2). Thus the entelechy of the body is the soul. The origin of the differentiation process is to be sought in a prime mover, i.e. pure form entirely separate from all matter, eternal, unchangeable, operating not by its own activity but by the impulse which its own absolute existence excites in matter.
The Aristotelian conception of form was nominally, though perhaps in most cases unintelligently, adopted by the Scholastics, to whom, however, its origin in the observation of the physical universe was an entirely foreign idea. The most remarkable adaptation is probably that of Aquinas, who distinguished the spiritual world with its subsistent forms (formae separatae) from the material with its inherent forms which exist only in combination with matter. Bacon, returning to the physical standpoint, maintained that all true research must be devoted to the discovery of the real nature or essence of things. His induction searches for the true form of light, heat and so forth, analysing the external form given in perception into simpler forms and their differences. Thus he would collect all possible instances of hot things, and discover that which is present in all, excluding all those qualities which belong accidentally to one or more of the examples investigated: the form of heat is the residuum common to all. Kant transferred the term from the objective to the subjective sphere. All perception is necessarily conditioned by pure forms of sensibility, i.e. space and time: whatever is perceived is perceived as having special and temporal relations (see Duration; Kant). These forms are not obtained by abstraction from sensible data, nor are they strictly speaking innate: they are obtained by the very action of the mind from the co-ordination of its sensation.
For example a cactus: