In the Physics Aristotle gives his famous definition of a substance, which he refers to as a thing that “exists by nature” or as a “natural object”:

Some things exist by nature, others are due to other causes. Natural objects include animals and their parts, plants and simple bodies like earth, fire, air, and water; at any rate, we do say that these kinds of things exist naturally. The obvious difference between all these things and things which are not natural is that each of the natural ones contains within itself a source of change and of stability, in respect of either movement or increase and decrease of alteration. On the other hand, something like a bed or a cloak has no intrinsic impulse for change — at least, they do not under that particular description and to the extent that they are a result of human skill, but they do in so far as and to the extent that they are coincidentally made out of stone or earth or some combination of the two.

The nature of a thing, then, is a certain principle and cause of change and stability in the thing, and it is directly present in it — which is to say that it is present in its own right and not coincidentally. (Physics II.1 192b8-b23)

Edward Feser summarises this definition from Aristotle by saying,

The basic idea, then, is that a natural object is one whose characteristic behavior — the ways in which it manifests either stability or changes of various sorts — derives from something intrinsic to it. (Between Aristotle and William Paley: Aquinas’ Fifth Way)

Aristotle and the Scholastics would later argue that the only way to make sense of the fact that things always, or for the most part, behave in certain ways is if they are by nature directed towards such behaviour. That is, if they have an inherent tendency or directedness towards such activity as an end. (cf Physics II.8 198b34-199a7) This intrinsic directedness towards an end, then, is the nature of thing:

The point is that those things are natural which undergo continuous change, starting from an intrinsic source of change and concluding at a particular end… it is clear that a thing’s nature is a cause, and that it is the kind of cause I have been saying — namely, purpose. (Physics 199b15-18, 32-33)

It must be recalled that neither Aristotle nor the Scholastics who followed him thought of this directedness or “purpose” as necessarily involving intelligence or deliberation from the things so directed.

This is particularly clear in the case of non-human animals, whose products are not the result of skill, enquiry, or planning. Some people are puzzled by how spiders, ants, and so on make what they make — do they use intelligence, or what? … It is ridiculous for people to deny that there is purpose if they cannot see the agent of change doing any planning. After all, skill does not make plans. If ship-building were intrinsic to word, then wood would naturally produce the same results that ship-building does. If skill is purposive, then, so is nature. (Physics II.8 199b26-30)

Again, Feser explains:

In other words, that goal-directedness does not require conscious deliberation is evident from the fact that a skilled craftsman can largely carry out his work without even thinking about it—”on autopilot” as we might put it today, or without first “making plans,” as Aristotle puts it. But if this is possible for someone with such skill, there is in Aristotle’s view no reason not to think it also possible for natural objects. This is the force of the ship-building example: If there were something in the very nature of wood that “directed it” toward the end of becoming a ship, then what in the case of human craftsmanship results from deliberate design — a ship — would in that case result “naturally” instead, that is, without conscious deliberation at all. Indeed, “it looks as though things happen at the plant level too which serve some purpose” in just this way, even though plants do not deliberate — for instance, an oak derives from an acorn without the acorn planning this result — and there is also of course the example of “non-human animals, whose products are not the result of skill, enquiry, or planning.”

So substances are those things which have an intrinsic directedness towards an end. Because this directedness is tied up with a thing’s characteristic behaviours, and characteristic behaviours are tied up with a thing’s causal powers, we might equivalently say that substances are those things which have intrinsic causal powers. By intrinsic, here, we mean that the directedness or causal powers of the thing are not (1) imposed from some outside agent or (2) reducible to the sum of the its parts considered in themselves. Aggregates (or “heaps”), on the other hand, have only extrinsic directedness or causal powers.

Let’s consider some examples of each. On a molecular level, a water molecule is a substance, for it has causal powers which are not reducible to the powers of its parts. For instance, water boils at 100°C while hydrogen, considered in itself, boils at -252.9°C and oxygen, considered in itself, boils at -183°C. The same goes for other powers.

On a more macroscopic level, individual animals are substances. Considered in itself, an organ is merely a clump of flesh which decomposes if left to its own devices. However, when the organs co-exist in an animal they are each capable of their individual functions in the body (walking, grasping, thinking, sensing, pumping blood, and so on) and they are all capable of participating in the life of the animal, where life is:

… the natural capacity of an object for self-perfective immanent activity. Living things act for themselves in order to perfect themselves, where by perfection I mean that the entity acts so as to produce, conserve and repair its proper functioning as the kind of thing it is… (David Oderberg, Teleology: Inorganic and Organic)

Consider, for instance, how you develop from a baby in your mother’s womb to a fully-grown adult, or how body heals itself when damaged, or how you don’t just decompose (unless you’re sick in some way). None of your organs, considered in themselves as mere clumps of flesh, are capable of these things and so you are not merely the sum of your organs.

A pile of rocks would be an obvious example of an aggregate. Its power to hold something 2 meters above the ground is merely the sum of the individual rocks that make it up. Above Aristotle used an example of a bed, which is merely an aggregate of the materials (wood and metal) that make it up.

Some aggregates, because of their complexity, are less obviously aggregates. Examples of these are things like watches and computers. A watch’s power for time-telling is imposed on it by us, and its power for the circular motion of its hands is merely the sum of the powers of its parts such as the conduction of electricity and so on. Similarly for a computer or a calculator.

From wholes to activities

All this is by way of introduction for what I really want to talk about here. The space was not wasted, however, for what we have introduced will serve us well in what follows. Thus far we’ve been discussing the distinction between substantial and aggregate wholes. My aim here, however, is to make a parallel distinction between substantial and aggregate activities.

A substantial activity, then, is one which has intrinsic directedness towards an end. That is, its directedness is not (1) imposed from some outside agent or (2) reducible to the sum of the its parts considered in themselves. In order for us to understand this we need to be clear on how an activity has directedness, and the best way to achieve such clarity is by considering how substances engage in activities. For our purposes here, it will be sufficient to distinguish between three groups of substances: non-animals, non-rational animals, and rational animals.

By non-animals I mean inorganic substances (rocks, water, atoms, …) and non-animal organisms (that is, vegetation). What distinguishes animals from non-animals is that the former have some form of sentience (and, typically, an ability for self-movement). Since non-sentience involves not being able consciously move to an end it seems we have two options with regards to how activities involving non-animals have directedness: either their activities don’t have directedness, or the directedness of their activities derives from the directedness the substance has in virtue of its nature.

What distinguishes rational animals from non-rational animals is that the former have the ability to (1) abstract universal concepts from particulars (“Socrates is a human“), (2) combine these concepts into judgements or propositions (“All humans are mortal”), and (3) string these propositions into arguments for conclusions (“Therefore, Socrates is mortal”).[1] So within animals we distinguish between non-rational animals, which are only conscious of particular things via sensation, and rational animals, which are additionally conscious of the universal concepts that pervade all the particulars. By virtue of their consciousness animals are capable of directing their actions towards specific ends in addition to the ends set for them by their natures.

For instance, a cat is by nature directed towards certain characteristic activities such as walking on four legs and eating certain types of food, as well as developing such morphological features that make these possible. However, because this cat is hungry and conscious of that bird it directs and moves itself towards that bird in order to eat it. All the while, however, the cat is not conscious of universal concepts (as such) like “being hungry”, “birds” and so on. Much of its “reasoning” is driven by instinct and nature. But this does not invalidate the claim that it has a measure of self-direction which it derives from its consciousness of particular things. Rational animals, because they are also aware of universal concepts, are capable of directing themselves in accordance with a richer set of ends.

Whether an animal is rational or not, ultimately its intention is what determines the direction of a given activity. Consider, for instance, the movement of my hand into your shoulder. What I intend to achieve with the movement is what determines whether this action is me punching you or merely an accident (which would be the case if I was intending to get something else and misjudged our relative positions).

With these distinctions in hand we ask the following question: how are substantial and aggregate wholes related to substantial and aggregate activities? It seems obvious that substances are capable of substantial actions and aggregates are capable of aggregate activity. But does this exhaust the possible relations?

The possibility of substantial activity by aggregate wholes

There are only two other options we could consider. The first is substances performing aggregate activities, but in the interest of time we’ll leave this to one side.

The second option is that of an aggregate performing a substantial activity. Such an activity would require an aggregate of substances to direct their otherwise disparate activity toward a common end. Is this not what we find in various teams and associations throughout human society? A sports team works together to win the game, an orchestra works together to play their piece well, the various employess in a company work together for the sake of the company, the various military personnel work together to achieve victory in war, when two friends pick up a large object together, and so on.

At this point we must be careful, lest we fall into error and think some such activities substantial when in fact they are merely aggregate. Take the example of the members of an orchestra performing their piece. This is an example of substantial action because each intends to contributes to the same performance. It’s this shared intention (or something like it) that makes the activity substantial, and not just that the sound produced is a combination of the sounds of the various instruments. After all, even aggregates involve combinations of their parts. By contrast, consider the musicians behind stage before the performance starts, while they each tune their respective instrument. An observer standing backstage will hear the combination of all the various sounds they make as they do this. The making of this combined sound will be merely an aggregate activity. Why? Because there is no shared intention directing the musicians to a common end. Rather, in this case each musician intends merely to tune their own instrument independent of the others. So when we aggregate the various tuning activities we end up with an aggregate of ends and therefore an aggregate activity.

So aggregate wholes can indeed engage in substantial activities, and they do so when and only when the members of the aggregate intentionally work together toward some common end.

But we can take this further. You’ll notice that the examples I listed above all involved aggregates of rational animals. This was not accidental, for only aggregates of rational beings are capable of this kind of substantial activity we’re considering. We see why when we reflect on what’s involved in working together with others toward a common end.

First, working together involves recognising both you and another falling under the same category of “part” in some sense. It requires that we understand the roles we’re responsible for and how those roles contribute to the achievement of the end in sight. Often (if not always) this will require that we understand the rules which encode our responsibilities. All of this, and more, requires a capacity for being conscious of universal concepts like “part”, “whole”, “role”, “responsibility”, “rule”, “expectation”, and so on. Since only rational beings are conscious of universal concepts, it follows that only rational beings can work together toward a common end.

Second, working together toward a common end requires that we be conscious of the end as common. Briefly, common ends are ends that can be enjoyed by multiple members without thereby being diminished. They are opposed to private ends, which are always diminished when shared.[2] For instance, if there is a loaf of bread between me and someone else, the more the I eat the less there is for the other person to eat. Siblings will know that the time I spend playing on the computer is time my brother cannot play on the computer. Consider, however, the examples we mentioned earlier: winning a sports game, the musical piece, the good of a company, victory in war, the picking up of a car. All of these are shared amongst the members in the corresponding aggregate, but are not thereby diminished. The same victory in war, for instance, is equally had by everyone in the winning nation.

Now, from the examples given it seems clear that particular things (or combinations of particular things) considered as particular can only serve as private ends. By contraposition, it follows that in order to be conscious of an end as common requires that we be conscious of universal concepts. Therefore only rational beings can be conscious of, and direct themselves toward, common ends.

Notes

1. The Scholastics called these the “acts of reason”, and labeled them (1) grasping, (2) composition and division, and (3) reasoning. Each of the acts of reason are dependent upon the earlier ones for their operation. Technically, (2) is richer than merely the ability to form propositions: it also enables rational beings to form universal concepts of things they haven’t experienced yet. For instance, once we have an concept of a horse and the concept of blackness we can consider the combination of these two concepts without having ever seen a black horse.
2. Of course this is not the whole story, and common ends tend to be notoriously difficult to talk about (see, for instance, Marcus Berquist’s Common Good and Private Good). The particular qualification I want to add here is that while common ends can be shared without thereby being diminished it doesn’t follow that sharing always leaves it undiminished. For instance, orchestras are limited in their size because once they get too big they become unmanageable. The same goes for political communities and friendships and presumably any community. Furthermore, including bad musicians in an orchestra might also diminish the end insofar as those musicians get in the way of the orchestra performing well. But in these cases it is not the sharing per se that is diminishing the end, but rather the sharing with too many people or sharing with bad musicians. With private goods, no matter how you share you will always diminish your ends. Because this qualification doesn’t affect the overall thrust of my argument, I chose to just mention it here in the footnotes.