External extension, then, is an absolute accident, really distinct from the corporeal substance, and naturally though not absolutely inseparable from the latter. It is the natural concomitant or consequence of the internal quantity whereby the corporeal substance has in itself a plurality of distinct integral parts. This internal quantity itself is either an aspect of the corporeal substance itself, only virtually distinct from the latter, or else in the strict sense a property, absolutely inseparable, if really distinct, from the substance. Natural experience furnishes no example of a corporeal substance actually existing devoid of internal quantity or internal distinction of integral parts.[364] But scholastic philosophers are not agreed as to whether the corporeal substance is itself and by its own essence a manifold of really distinct integral parts (in which case internal quantity would be merely the aspect under which the essence is thus regarded as an integral whole constituted by a plurality of distinct integral parts; while, looked at as an essence, it would be an essential whole constituted by the union of two essential parts or principles)—or whether it is formally constituted an integral whole, not by its essence (which makes it only an essential whole, an essentially composite substance), but by a property really distinct, though necessarily flowing, from this essence, viz. internal quantity. According to the former view the material principle (materia prima) of the composite corporeal substance is such that the essence resulting from its union with the formative principle (forma substantialis) is necessarily an integral whole with distinguishable integral parts, each of which naturally demands the spatially extended mode of being which external extension de facto confers upon it. According to the latter view, which is that of St. Thomas and his followers generally, the corporeal substance as such has no mode of composition other than essential composition: it is not of itself an integral whole, compounded of distinct or distinguishable integral parts (each of which would be, like the whole, essentially composite): of itself it is indivisible into integral parts: it is, therefore, in [pg 315] this order of being, simple and not composite. It has, no doubt, by reason of its material principle, an absolutely necessary exigence for divisibility into distinct integral parts, for integral composition in other words. But this actual integral composition, this actual divisibility, is the formal effect of a property really distinct from the substantial essence itself; and this property is internal quantity: the connatural, but absolutely separable, complement of this internal quantity being, as in the other view, local or spatial extension.
In both views external extension is an absolute accident of the corporeal substance; and in the Thomist view internal quantity would also appear to be an absolute accident, and not a mere mode.
It is instructive to reflect how far this scholastic doctrine removes us from the Cartesian view which sets up an absolute antithesis between mind or spirit, and matter or body, placing the essence of the former in thought and that of the latter in extension. According to the scholastic view the spiritual substance is an immaterial “actuality” or “form”; it is essentially simple, and not like a corporeal substance an essentially composite substance resulting from the union of a formative principle or “form” with a passive, determinable, material principle. And since it is the material principle that demands the property of internal quantity and the accident of external extension, whereby the corporeal substance becomes an integral whole with its parts extended in space, it follows that the spiritual substance, having no material principle in its constitution, is not only essentially simple—to the exclusion of distinct principles of its essence,—but is also and as a consequence integrally simple, to the exclusion of distinct integral parts, and of the extended or characteristically corporeal mode of occupying space. So far there is contrast between the two great substantial modes of finite being, matter and spirit; but the contrast is by no means an absolute antithesis. For if we look at the essence alone of the corporeal substance it is not of itself actually extended in space: in the Thomist view it is not even of itself divisible into distinct integral parts. It differs from spirit in this that while the latter is essentially simple the former is essentially composite and has by reason of this compositeness a natural aptitude for divisibility into parts and for the extension of these parts in space, an aptitude which spirit does not possess. But the corporeal substance may exist without actual extension, and consequently without any of those other attributes such as impenetrability, solidity, colour, etc., through which it is perceptible to our senses. In this condition, how does it differ from spirit? In being essentially composite, and in being perhaps endowed with distinguishable integral parts.[365] But in this condition the [pg 316] essential mode of its being has a relation to space which closely resembles the mode in which spirit exists in space: it is related to space somewhat in the manner in which the soul is in the space occupied by the body—whole in the whole of this space and whole in every assignable portion of this space. So that after all, different as matter and spirit undoubtedly are, the difference between them is by no means that sort of Cartesian chasm which human thought must for ever fail to bridge.
By virtue of its external extension the corporeal substance exists by having distinguishable parts outside parts in space. We can conceive any perceptible volume of matter as being perfectly continuous, if it has no actual limits or actual distinction of parts within itself, but is one individual being completely filling the whole space within its outer surface; or imperfectly continuous, if while being one and undivided it has within its volume pores or interstices, whether these be empty or filled with some other sort of matter; or as made up of contiguous integral parts if each or these is really distinct and actually divided from every other, while each actually touches with its outer limits the adjacent limits of the parts lying next to it, so that all the internal parts or limits are co-terminous; or as made up of separate, discrete or distant parts no one of which actually touches any other.
It is clear that there must be, in any actually extended volume of matter, ultimate parts which are really continuous—unless we are to hold, with dynamists, that our perception of extension is produced in our minds by the action of extramental points or centres of force which are themselves simple or unextended. But the physical phenomena of contraction, expansion, absorption, undulatory and vibratory motions accompanying our sensations of light, heat and sound, as well as many other physical phenomena, all point to the fact that volumes of matter which are apparently continuous are really porous: the molecular structure of perceptible matter is an accepted physical theory; and scientists also universally accept as a working hypothesis the existence of an imperceptible material medium pervading and filling all real space, though there is no agreement as to the properties [pg 317] with which they suppose this hypothetical medium, the “ether,” to be endowed.
Again, as regards the divisibility of extended matter, it is obvious that if we conceive extension in three dimensions geometrically, mathematically or in the abstract, any such volume or extension is indefinitely divisible in thought. But if we inquire how far any concrete, actually existing volume of matter is divisible, we know in the first place that we cannot divide the body of any actual organic living thing indefinitely without destroying its life, and so its specific character. Nor can we carry on the division of inorganic matter indefinitely for want of sufficiently delicate dividing instruments. But apart from this the science of chemistry points to the fact that every inorganic chemical compound has an ultimate individual unit, the chemical molecule, which we cannot sub-divide without destroying the specific nature of the compound by resolving it into its elements or into less complex compounds. Furthermore, each “elementary” or “chemically simple” body—such as gold, oxygen, carbon, etc.—seems resolvable into units called “atoms,” which appear to be ultimate individual units in the sense that if their mass can be subdivided (as appears possible from researches that have originated in the discovery of radium) the subdivisions are specifically different kinds of matter from that of the atom so divided.
In the inorganic world the perceptible mass of matter is certainly not an individual being, a unum per se, but only a collection of individual atoms or molecules, a unum per accidens. Whether the molecule or the atom of the chemically elementary body is the “individual,” cannot be determined with any degree of certitude. It would appear, however, that every specifically distinct type of inorganic matter, whether compound or elementary, requires for its existence a certain minimal volume, by the sub-division of which the type is substantially changed; and this is manifestly true of organic or living matter: so that matter as it naturally exists would appear not to be indefinitely divisible.
If in a chemically homogeneous mass of inorganic matter (such as carbon or water) the chemical molecule be regarded as the “individual,” this cannot be the case in any organic, living thing, for whatever matter is assimilated into the living substance of such a being eo ipso undergoes substantial change whereby it loses the nature it had and becomes a constituent of the living individual. The substantial, “individual” unity of the organic living being seems to be compatible not merely with qualitative (structural and functional) heterogeneity of parts, but also with (perhaps even complete) spatial separateness [pg 318] of these parts. If the structure of the living body is really “molecular,” i.e. if it has distances between its ultimate integral units, so that these are not in spatial contact, then the fact that the formative, vital principle (forma substantialis, anima) unifies this material manifold, and constitutes it an “individual” by actualizing and vitalizing each and all of the material units, spatially separate as they are,—this fact will help us to realize that the formative principle of the composite corporeal substance has not of itself the spatial, extended mode of being, but that the substance derives the latter from its material principle (materia prima).
84. Place and Space.—From the concept of the volume or actual extension of a body we pass immediately to that of the “place” (locus) which it occupies. We may distinguish between the internal and the external place of a body. By the former we understand the outer (convex) surface of the body itself, regarded as a receptacle containing the volume of the body. If, therefore, there were only one body in existence it would have its own internal place: this is independent of other bodies. Not so, however, the external place; for by the external place of a body we mean the immediately surrounding (concave) surface, formed by the bodies which circumscribe the body in question, and considered formally as an immovable container of this body. This is a free rendering of Aristotle's definition: Place is the first (or immediate) immovable surface (or limit) of that which contains a body: prima immobilis superficies ejus quod continet.[366] If a hollow sphere were filled with water, the inner or concave surface of the sphere would be the “external place” of the water. Not, however, this surface considered materially, but formally as a surface, so that if the sphere could be removed, and another instantaneously substituted for it, the water would still be contained within the same formal surface; its locus externus would remain the same. And, again, it is the containing surface considered as immovable or as circumscribing that definite portion of space, that constitutes the locus externus or “external place” of the located body: so that if the sphere with the water were moved the latter would thereby obtain a new external location, for though the containing surface be still materially and formally the same, it is no longer the same as a locating surface, seeing that it now marks off a portion of space different from that marked off by it before it was moved.
Aristotle's definition defines what is known as the proper external place of a body. From this we distinguish the common [pg 319] external place or location of a body: understanding by the latter, or “locus communis,” the whole collection of spatial relations of the body in question to all the bodies in its immediate neighbourhood. It is by indicating these relations, or some of them, that we assign the Aristotelian category, or extrinsic denomination, Ubi.[367]