What is it to Be a Material Substance?
Whatever else a material substance is supposed to be, it is the sort of thing subject to becoming and limited location. It is, after all, by their changeable character that things in the physical world first come to be understood; and it is by their ability to stand in relation to some point in space-time that we can testify to a multiplicity of things, as opposed to a singular reality.
What does it mean to say that material substances undergo a process of becoming? In the Aristotelian sense, it is simply to say that they undergo change of various sorts, where “change” is to be understood in terms of a thing potentially realizing some perfection, and actually realizing the perfection when acted upon by some cause. Any account of the behavior, reaction, activity, growth and other features of material substances will fundamentally involve what the Scholastics referred to as a reduction (or transition) from potentiality to actuality, from a thing’s having an inherent capacity for X to X being brought about by some cause which has the power to bring it about.
Consider the following examples of becoming:
1) The potential for a balloon to increase in surface area is actualized by the amount of gas one pumps into it. Change of this kind — i.e. involving increase and decrease — is known as quantitative change.
2) The potential for wood to become hot is actualized by its coming into contact with fire. Change of this kind — i.e involving alteration — is known as qualitative change.
3) The potential for the pool ball to move from one location to another is actualized by the motion imparted to it by the pool cue. Change of this kind — i.e with respect to place — is known as locomotive change.
4) The potential for a lump of bronze to be turned into a statue is actualized by the activity of the sculptor. Change of this kind — i.e. involving the coming to be of one form and the passing away of another — is known as substantial change.
Note that every one of these examples presuppose a temporal framework. Anything coming into being, passing away, and changing with respect to movement, quality, or quantity, will entail a temporal succession between some initial time A — the water in the teapot being cold; a lump of bronze — and a final time Z — the water in the teapot actually becoming hot; the lump of bronze actually becoming a statue. Put differently, there is a time at which some perfection is only potentially present in the subject; there is another time at which that thing’s perfection is actually realized in the subject. This principle holds true even in those instances where causes and effects are taken to operate simultaneously: the train pulling the caboose, and the caboose being pulled by the train are one and the same effect (considered under different descriptions); nevertheless, there is a time at which the train is not imparting motion to the caboose, nor the caboose being moved by the train.
Finally, what does it mean for an object to be subject to limited location? It means simply that things are contingent with respect to place. A billiard ball occupies a certain position C in space, although it may conceivably occupy position A, or B, or D, and so on. A cluster of stars occupies a certain, quantifiable point of reference in our galaxy, although it may have conceivably occupied a different point, even as part of a different galaxy. Examples abound, but the point is simply that a material substance is limited to a narrow range of possibilities within space-time.