EXTENSION (Lat. ex, out; tendere, to stretch), in general, the action of straining or stretching out. It is usually employed metaphorically (cf. the phrase an “extension of time,” a period allowed in excess of what has been agreed upon). It is used as a technical term in logic to describe the total number of objects to which a given term may be applied; thus the meaning of the term “King” in “extension” means the kings of England, Italy, Spain, &c. (cf. Denotation), while in “intension” it means the attributes which taken together make up the idea of kinghood (see Connotation). In psychology the literal sense of extension is retained, i.e. “spread-outness.” The perception of space by the senses of sight and touch, as opposed to semi-spatial perceptions by smell and hearing, is that of “continuous expanse composed of positions separated and connected by distances” (Stout); to this the term “extension” is applied. The perception of separate objects involves position and distance, but these taken together are not extension, which necessarily implies continuity. To move one’s finger along the keys of a piano gives both the position and the distance of the keys; to move it along the frame gives the idea of extension. By expanding this idea we obtain the conception of all space as an extended whole. To this perception are necessary both form and material. It should be observed the actual quality of a stimulus (rough, smooth, dry, &c. ) has nothing to do with the spatial perception as such, which is concerned purely with what is known as “local signature.” The elementary undifferentiated sensation excited by the stimuli exerted by a continuous whole is known as its “extensive quantity” or “extensity.” The term has to do not with the kind of object which excites the sensation, but simply with the vague massiveness of the latter. As such it is distinguishable in thought from extension, though it is not easy to say whether and if so how far the quantitative aspect of space can exist apart from spatial order. Extensity as an element in the complex of extension must be carefully distinguished from intensity. Mere increase of pressure implies increase of intensity of sensation; to increase the extensity the area, so to speak, of the exciting stimulus must be increased. Thus the extensity (also called “voluminousness,” or “massiveness”) of the sensation produced by a roll of thunder is greater than that produced by a whistle or the bark of a dog. It should be observed that this application of the idea of extensity to sensation in general, rather than to the matter which is the exciting stimulus, is only an analogy, an attempt to explain a common psychic phenomenon by terminology which is intrinsically suitable to the physical. As a natural consequence the term represents different shades of meaning in different treatises, verging sometimes towards the physical, sometimes towards the psychic, meaning.

In connexion with extension elaborate psycho-physical experiments have been devised, e.g. with the object of comparing the accuracy of tactual and visual perception and discovering what are the least differences which each can observe. At a distance two lights appear as one, just as two stars distinguishable through a telescope are one to the naked eye (see Vision): again if the points of a compass are brought close together and pressed lightly on the skin the sensation, though vague and diffused, is a single one.

See Psychology and works there quoted; also Space and Time.