134. The principal properties common to all bodies and to no other substances are extension, impenetrability, figure, local motion, and inertia.
I. Extension is continuous quantity, or that property of a body by which it has parts outside of each other and so united by a common bond as to constitute a physical unit. That this property really exists in bodies we know for certain by the testimony of our senses. Still, the essence of bodies does not consist in extension, as Descartes supposed; for space, too, has extension, and yet it is not a body. Extension is natural to bodies, but it is not, therefore, essential to them. True, every body has parts distinct from each other; besides, quantity gives parts a tendency to be outside of one another; next, this tendency is actuated if not divinely impeded. But it may be divinely impeded; and when this happens, as it does with regard to the Body of our Lord in the Holy Eucharist, the parts of the body are not outside of each other; but the sacred Body of Christ is present, after the manner of a spirit, being whole in every part of the species.
135. II. Impenetrability is the property by which one body excludes another from the place it occupies. It is natural to bodies, and the tendency thus to exclude other bodies is even essential to bodies; still, its effects can be suspended by the Almighty, as can all action of any created substance. In such a case two or more bodies could occupy the same place, as when the risen Saviour entered the Upper Room, though the doors were closed. On the other hand, it is not absolutely impossible, because not self-contradictory, that one body should by reduplication of its relations, be in two or more places at the same time.
136. III. Figure is an accident of continuous quantity which results directly from its limitations; for whatever is limited must have definite limits; these definite limits to extension constitute figure. The natural figure of a body is determined by its substantial form; for the form gives to the body every determination that belongs to such a species, though its effects in individual beings are influenced by present circumstances. Thus, the forms of metals determine the figures of their crystals; so with plants and animals; still, circumstances may favor or impede the action of the form in each individual case, as when a plant is dwarfed in a cold climate.
137. IV. Local motion is the successive transition of a thing from one place to another. It supposes a subject, a term to which the subject tends, a force impelling it to that term. If only one body existed, there would be no such a term, and therefore no motion in the strict meaning of that word, though an impelling force might exist, and cause in that body a mode which might analogically be called motion. Local motion, then, implies a change of place; but spirits are not in a place, in the same sense that bodies are in a place, and therefore motion is not predicated of them in the same sense. Spirits, as such, have no necessary relation to matter, and may have been created before matter; but place and local motion are properly accidents of matter only, and therefore cannot affect spirits except indirectly, inasmuch as spirits may be in union with a body, either substantially, as the soul of a living man, or virtually, as when an Angel protects a child. (See St. Thomas, Summa, i. q. lx. 4, Ium) In this latter case he exhibits his presence in space and acts in space, but is not limited by space.
138. V. Inertia does not mean that bodies, as such, have no powers to act, but that they cannot of their own accord begin to act or cease to act, or make any change in their mode of acting. The reason of this impotence lies in the fact that bodies, as such, i.e., as mere material substances not informed by a vital principle, have no perceptions, and therefore no motives to determine their actions in one way rather than another; therefore they can only act uniformly and without spontaneity or power of self-determination. Intelligent and sentient beings, on the contrary, can know a term to which they may tend, and therefore determine themselves to motion or rest, or to one motion rather than another. Intelligent beings may do so freely, because they apprehend the term as unnecessary; but those that are merely sentient, as brutes are, act necessarily upon the stronger attraction; still, their actions, unlike those of non-sentient things, are determined by their instinct, which is a principle intrinsic to themselves.