Extension (from Lat. extendere, to spread out).— That material substance is not perfectly continuous in its structure, as it appears to our gross senses, the physical sciences demonstrate. The microscope reveals pores in the most compact matter, while the permeation of gases and even of liquids through solids indicates that the densest bodies would probably present to a sufficiently penetrating eye a sponge-like structure throughout. This fact, together with the difficulty of explaining how the senses can perceive extension, has led many theorists to deny its objectivity, although, on the other hand, the first of modern philosophers, Descartes, was so impressed by the universality of extension that he held it to be the very essence of matter. Kant makes extension a subjective form, an original condition of sensuous faculty which when stimulated by the sense-object stamps the impression accordingly. Others, with Leibniz, resolve matter into simple unextended points (monads), which by their agitation are supposed to produce in us the impression of continuous extension. Others, with Boscovich (d. 1787), subtilize matter into simple forces which some hold to be "virtually" extended. The Atomists (physical and chemical) dissolve bodies into minute particles or atoms (which some consider to be absolutely, others only physically, indivisible) of certain elementary substances, which hitherto have defied further analysis but which may eventually turn out to be merely varying arrangements of some primordial homogeneous material, the radical constituent of the universe. The present teaching of Catholic philosophy on the subject may be summarized as follows: Extension is either successive (fluent, as that of a stream and of time), or permanent. The latter may be viewed as either (a) continuous (mathematical, i.e. abstract, as a line; or physical), when the entitative or integrant parts into which its immediate subject, material substance, is divisible are united (perfectly or imperfectly) throughout, e.g. a homogeneous wire; (b) contiguous, when the said parts are conjoined only by contact, e.g. a brick wall; (c) interrupted, when those parts are in some degree disjoined, though connected by an intermediate, e.g. a string of beads. We are here occupied with continuous extension only.

Continuous extension may be described as that property in virtue whereof the parts into which material substance is divisible are situally arranged in orderly relation one beyond the other (internal and potentially local extension) and hence are naturally commensurate with the corresponding parts of the immediately environing surfaces (external and actual local extension). Consequent attributes of extension are divisibility, measurability, and impenetrability. Wherein precisely the essence of extension consists, is a controverted question. Probably the more general opinion is that extension radically and essentially consists in the internal distribution of the parts into which matter is divisible, and that external extension, or the correspondence of those parts to the parts of the locating surfaces, is a sequent property of essential or internal extension. Of course this does not explain extension. Some nearer approach to an explanation may be found in the opinion of a recent writer (Pecsi) who makes extension consist in the expansive and cohesive forces of matter—the former causing the said parts to spread out, the latter keeping them united.

Continuous extension is an objective property of matter, not a mere mental form moulding the sensuous impression produced in the sensory organs by some sort of physical motion. What it is that extension immediately affects—whether the ultimate atoms, the constituent molecules, or the gross mass of matter—we are unable in the present stage of physical science to decide. Even should it turn out, however, as many conjecture, that the densest solid—to say nothing of a liquid or a gas—is but what might be called an "infinitely" complex arrangement of infinitesimal corpuscles—atoms or electrons-gyrating in a matrix of ether, continuous extension would still remain real (objective), though it would then be the immediate property of the constituent corpuscles and the ether instead of a property of the gross mass. It is experimentally demonstrable that sensuous impressions are aroused in us by bodies as extended and resistent. Now if bodies were constituted of simple, unextended points—monads or forces—these could not stimulate the sensory organs, since such elements, apart from the fact that they would all coalesce and copenetrate, could not be the subjects of material activity (etherial or aerial vibrations, chemical reactions, i.e. the immediate sense-stimuli). Nor could the organs evoke the sensation, since in the hypothesis they, too, being made up of unextended elements, would be incapable of material action. Neither will it do to say that the motion of the supposed "points" might evoke sensation, since being unextended they would be imperceptible whether in motion or at rest.

Extension is an "absolute accident", that is not a mere mode in which substance exists, as, for instance, are motion and rest. It seems to have a certain distinct entity of its own. This, of course, would most probably never have been suspected by the human mind unaided by Revelation. But given the doctrine of the Real Presence of Christ in the Sacrament of the Eucharist, wherein the extensional dimensions and sensible qualities of bread and wine persist after the conversion of the substance of the bread and wine into His Body and Blood, reason, speculating on the doctrine, discerns some grounds for the possibility of the real distinction and even severance between substance and local extension. In the first place there are motives for inferring a real distinction between substance and extension (actual and local), or, in other words, that extension does not constitute the essence of material substance (as Descartes maintained that it does): (a) substance is the root principle of action; extension as such is either inactive or at most a proximate principle; (b) substance is the ground of specification; extension as such is indifferent to any species, since shape or figure which is the dimensional termination of extension depends upon the specific form; (c) substance is identical in the entire mass and in each of its parts (e.g. in gold), while extension is not the same in the whole and in each of its parts; (d) substance is the principle of unity; extension is the formal principle of plurality; (e) substance essentially demands three dimensions; extension may be realized in one or two; (f) substance remaining the same, extension may increase or decrease.

Given a real distinction between extension and substance, no intrinsic impossibility can be proven to exist in the separation of one from the other, for although internal extension naturally demands external, there is no evidence that the demand is so essentially imperative that Omnipotence cannot supernaturally suspend its realization and by other means afford the accidents—extension and the rest—the support which the substance naturally supplies. Since material substance owes the distribution of its integral parts to extension, the question arises whether, independently of extension, it possesses any such parts (it, of course, possesses parts essential to corporeal substance, matter and form), or is simple, indivisible. St. Thomas and many others maintain that substance as such is indivisible. Suarez and others hold that it is divisible. For this and the other questions concerning the divisibility of extension, and the psychology of the subject, the reader is referred to the works mentioned below.

F. P. SIEGFRIED