(From Latin ex-tendere , 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
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):
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