Aristotle's Many Lives

In this passage, Aristotle specifies the material and efficient cause of the activity of nutrition:

Seeing that there are three things, what is nourished, what it is nourished by, and what nourishes, what nourishes is the primary soul, what is nourished is the body that has it [i.e., the body that has the primary soul], and what it is nourished by is the nourishment. (But since it is right to address everything based on the end, and generating one of the same sort as itself is an end, the primary soul would be one generative of one of the same sort as itself.) ⁽¹⁰⁾ But what it is nourished by is twofold, just like what a ship is steered by, namely, the hand and the rudder – the one causing movement and being moved, the other being moved only. For all nourishment must be capable of being concocted, and what effects the concoction is heat, which is why every ensouled thing has heat.just like what a ship is steered by, namely, the hand and the ⁽¹¹⁾

Aristotle's distinction between ‘three things’ involved in this activity is based on his discussion of motion in general in Physics VIII.5 where he distinguishes between the primary mover, the instrument(s) by which the primary mover moves something, and that which is ultimately moved. The primary mover moves something without itself being moved, while the instruments move something insofar as they are themselves moved by the primary mover. It is tempting to think of this as a way of describing two discrete processes (i.e., the primary mover moves the instruments and the instruments in turn move some other object), but this is exactly what Aristotle wants to avoid. It is rather that the action of the instruments is only the kind of action it is in virtue of being the way the primary mover acts on what is ultimately moved. Aristotle's discussion is relevant in our De Anima passage because it indicates the unity of the process by which the soul uses the vital heat of the body and the nourishment it receives to actually nourish the body. ⁽¹²⁾

In this passage, Aristotle specifies the final cause of the activity of nutrition, distinguishing it at the same time from the activities of growth and generation:

But since nothing is nourished that does not partake in soul, what is nourished will be the ensouled body, insofar as it is ensouled, so that nourishment is also relative to an ensouled being – and not accidentally so. The being for nourishment, however, is distinct from the being for a growth-producer. For it is growth-producing insofar as the ensouled thing is a certain quantity, whereas it is nourishment insofar as the ensouled being is a this something [tode ti] and a substance [ousia]. For the nourishment preserves the substance and the substance exists just as long as it is nourished, and nourishment is generative not of what is being nourished, but of something of the same sort as what is being nourished. For the substance of the body being nourished already exists, and nothing generates itself, but rather preserves itself. So, this sort of starting-point of the soul is a capacity of the sort that preserves what has it, insofar as it is ensouled, and nourishment equips it to be active. That is why, if deprived of food it cannot exist. ⁽¹³⁾

Here Aristotle is characterizing the ways in which one and the same edible object coming into the body can be alternately (i) nutritive (ii) growth-producing or (iii) generative, depending on whether this object is used to act on the living body (i) insofar as that body is a substance and hence is used to preserve that body in being or (ii) insofar as that body is of certain dimensions and hence is used to increase those dimensions or (iii) insofar as that body is of a certain species and hence is used to generate another of that same species.

What it is to be nourishment then is to be an instrument that the soul uses for the end of preserving the ensouled body, enabling it to be active. But is that all there is to being nourishment? I think not, since Aristotle's only intention in this passage is to distinguish how this one and the same object can be nutritive or growth-producing or generation-enabling, depending on what the soul uses it for. Yet in order to be any of these, this object needs to have undergone the biochemical process of concoction, and having undergone this process seems to be essential to anything's being nourishment. ⁽¹⁴⁾

At this point, I want to engage with a criticism interpreters often make of Aristotle's account of nutrition: one cannot define life in terms of nutritive activity because Aristotle defines nutritive activity in terms of what's alive. ⁽¹⁵⁾ Hence, Aristotle can only give a circular definition of life. The objector says the following: ‘What distinguishes the activities of a fire's being fed more wood and an organism's being fed? The only relevant distinction is that organisms are alive and fires aren't. Hence, one will have to define the kind of feeding organisms engage in as an activity living things engage in. That means we can't then define being alive in terms of engaging in nutritive activity.’

To respond to this objection, we have to keep in mind Aristotle's distinction between two ways of being more knowable: what is more knowable relative to us and what is more knowable relative to nature. ⁽¹⁶⁾ Greater knowability relative to us means what is better known at the outset of our inquiry, generally what is apparent to us through observation. Greater knowability relative to nature means what is really more fundamental and which causally explains more posterior facts. Hence, that tuberculosis impairs one's breathing is more knowable relative to us, insofar as it is something more readily discovered through observation. However, that a microorganism Mycobacterium tuberculosis causes tuberculosis is more knowable relative to nature because it is what explains the effects of the disease. Hence, the entire project of science is just using what is more knowable relative to us to discover what is more knowable relative to nature and then to explain using these more fundamental facts that which is more obvious to us.

In the case of life and the soul, we need to recognize that life is more knowable relative to us, whereas the soul – the principle whereby something is alive – is more knowable relative to nature. It is the soul that explains why something is alive. Hence, when Aristotle defines nutrition in terms of the activity of the soul on an ensouled body, this is compatible with his thinking that one can then define life in terms of nutrition. For even though being ensouled and being alive are coextensive, a substance's being ensouled is what explains its being alive.

Before getting into Aristotle's rejection of the possibility of a univocal definition of soul, I should summarize his basic account of univocity and equivocity (also called ‘synonymy’ and ‘homonymy’ from the Greek terms). In the Categories Chapter 1, Aristotle characterizes two things as univocal (synonymous) in virtue of their having both a name and the definition [logos] specifying the essence [ousia] denominated by that name in common. Two things are equivocal (homonymous) in virtue of their having only their name in common, while the definitions specifying the essence denominated by that name are different.

Some distinctive features of Aristotle's account of univocity stand out. He is describing a relation between actual things in the world rather than words or concepts, but this relation obtains between these actual things on the basis of both linguistic and metaphysical facts. So, dogs and cats are synonymous in virtue of their having a name in common (i.e., ‘mammal’) and this name's having the same essence-specifying definition in each case, i.e., a cat is a mammal in virtue of being an animal which belongs to a species that produces milk just as a dog is a mammal in virtue of being an animal which belongs to a species that produces milk. On the other hand, even though the arms on one's body and the arms one uses in war have a name in common (i.e., ‘arms’), the essence-specifying definition for each will be different. In the one case, some things are arms in virtue of being a kind of limb for an animal, while in the other, some things are arms in virtue of being weapons. As we will see in Aristotle's discussion of souls and lives, the relation between equivocal (homonymous) terms is often closer than that between the two kinds of arms. In other words, equivocity is compatible with there being a non-accidental relation between two items, but what it doesn't allow is for there to be a complete overlap in their essence-specifying definitions in relation to the name they share in common. ⁽¹⁸⁾

Aristotle's rejection of a univocal definition of souls and lives is based on (i) the similarity between how different kinds of geometrical figures are related to each other and how different kinds of souls are related to each other and (ii) the impossibility of giving a univocal definition of geometrical figures.

It is clear, then, that there could be a single account of soul in the same way as there is of figure. For in the latter case there is no figure beyond the triangle and the others that are next in sequential order, and in the former there is no soul beyond the ones we mentioned. In the case of the figures, though, we could produce a common account, which would fit them all and be special to no figure. Likewise, too in the case of the souls we mentioned. That is why it is ridiculous to inquire after a common account both in these cases and in others, when it is an account that will not be special to any being, and will not accord with its proper indivisible species, to the neglect of the one that is of this sort. ⁽¹⁹⁾

At first glance, one might wrongly take Aristotle to be making a point familiar from his metaphysics of species and genus: no genus can exist independently of the species which specify it. It is not possible for something to exist as an animal without existing as some particular species of animal. In contemporary metaphysics, one might say that the genus is a determinable which only exists insofar as it is determined to a certain species. Yet the comparison with figures suggests that this is not the point Aristotle is making. Rather what is distinctive about both geometrical figures and souls (and numbers too) is that there are priority and posteriority relations among the different kinds. While there are clearly priority and posteriority relations between the species in relation to the genus, there are no such priority relations among the species themselves. In particular, when defining a dog, one must define it in terms of its genus, and in order for the genus animal to exist, some member of a species of animal must exist. ⁽²⁰⁾ However, when defining a pentagon, one must define it not in terms of a genus but in terms of how it is constructed – which construction will itself depend on the construction of triangles. ⁽²¹⁾ So, in Euclid's Elements, the theorem for inscribing a pentagon in a given circle requires one to construct triangles (Book IV, Proposition 11). While the definition of the triangle is among Euclid's axioms, other figures like pentagons and hexagons he constructs in the course of his theorems. Indeed, it is critical that he proceed this way to actually demonstrate the various properties of these figures. ⁽²²⁾

Christopher Frey gives the following argument to show why souls can't belong to a single genus given that they are ordered by priority relations:

A genus can exist even if any one given species under it does not exist.

Frey's argument is based on a modal claim about the possibility of the first member of a series existing without the other members in this series. Aristotle's point seems to go further than this because it is not merely that souls with only the nutritive power can exist even without the other powers but that it is essential to the higher kinds of souls that they have such a nutritive power. ⁽²⁵⁾

In the passage that follows, Aristotle makes these priority relations among different kinds of figures and souls explicit:

For the situation as regards the soul is quite similar to the one where the figures are concerned. For what is prior is always potentially present in what is next in order, both in the case of the figures and in the case of the animate things – for example, the triangle in the quadrilateral, and the nutritive part in the perceptual one. So, we must inquire as regards each given case what the soul of each given one is – for example, what that of a plant is and that of a human or a beast. ⁽²⁶⁾

As we will see in the next section, it is critical that one recognize how more advanced souls potentially contain more primitive souls inside of them, i.e., the powers of these more primitive souls are possessed by whatever has the more advanced souls. If one doesn't recognize this potential containment, one will fail to seek explanatory connections among different powers in the soul. Imagine that one denied that there were such priority relations among different kinds of souls. In that case, one wouldn't be in a position to investigate why it is that certain powers of the soul can exist on their own, whereas others presuppose certain other powers. ⁽²⁷⁾

At this point, I turn to two prominent interpreters of Aristotle's definition of life – Gareth Matthews and Christopher Shields – and show how their opposed interpretations both fail to capture the kind of non-univocal definition of life Aristotle gives here in De Anima II.3.

In interpreting Aristotle's definition of life, Gareth Matthews proposes that there is a common attribute all life functions and their corresponding powers have in common. In particular, he defines these powers for different life functions (what he calls ‘psychic powers’) in the following terms:

x is a psychic power for species s = _df for s to be preserved individual organisms that belong to s must, in general, exercise x. ⁽²⁸⁾

Then, Matthews can define some substance's being alive in the following terms:

x is alive = _df there is a species s, and a psychic power p, such that x belongs to s, p is a psychic power for species s, and x can exercise p. ⁽²⁹⁾

As Matthews admits, the definitions will involve circularity, since what it is for an individual organism to be preserved is for its life to continue. ⁽³⁰⁾ Hence, one will define life in terms of psychic powers and psychic powers in terms of an organism's preservation and this preservation in terms of life.

Matthews' response to this circularity objection is unconvincing, but as an interpretation of Aristotle's quite distinctive way of defining life and the soul in De Anima II.3, it is even less promising. For according to Matthews' interpretation, Aristotle is really offering a univocal definition of life after all. What it is to be alive is to belong to a species and to have a psychic power for that species, and this applies equally to sponges, dogs, humans, and even (as Matthews says) gods. ⁽³¹⁾

Another interpretation is provided by Shields himself, who takes Aristotle to give the following account of what it is to be alive:

x is alive = _chdf x is an intentional system ⁽³²⁾

Shields defines an intentional system as ‘a system whose nature is accurately explained and whose behaviour is reliably predicted by understanding it as engaging in certain forms of end-directed behaviour’ and ‘whose activities can be assessed in part by the degree to which it attains its goals, and indeed a system whose goals provide a normative standard of its being a good instance of its kind.’ ⁽³³⁾ Later, Shields clarifies that life requires that x be a ‘native’ intentional system (as opposed to a ‘non-native’ intentional system) where a native intentional system is one whose ‘ends are not derived from the designing activities of external agents.’ ⁽³⁴⁾ One distinguishing mark of native versus non-native intentional systems is that only for the former is it intelligible to speak of something's being good for that system. E.g., it's intelligible to say that it's good for the ficus that it be watered but not intelligible to say that it's good for the computer that it be cooled down.

One problem with Shields' proposal is that it's still unclear how he avoids ascribing a univocal definition of life to Aristotle. To respond to this worry, Shields says that there are different ways of being an intentional system in virtue of there being different ends towards which different things' behavior is directed. On its own, this doesn't answer the worry because this kind of variety is compatible with univocity. For instance, there are different ways of being an animal, but animal in general can be defined univocally. What Shields needs is for there be an intentional system which is the primary way of being an intentional system, and for other intentional systems to be intentional systems only in virtue of being appropriately related to this primary way of being an intentional system.

In fact, Shields' ultimate proposal is just this: God is the primary intentional system, and other living things are intentional systems only in virtue of God's bearing a formal-causal relation to them. I will set aside the difficult question of whether it is possible for there to be a formal cause which is not univocal with what it is the formal cause of. ⁽³⁵⁾ Whatever the merits of such a proposal, it cannot be how Aristotle defines life. Indeed, it seems to be nearly the opposite of Aristotle's view, which is really a kind of ‘bottom-up’ view of life and not like Shields' ‘top-down’ view. For on Aristotle's definition, one defines life by first defining the most common life-function: nutrition. Then one defines increasingly advanced forms of life in terms of higher life-functions, but which in each case presuppose the presence of nutrition. Souls form a series, but the first member of this series isn't God's soul but rather the soul of plants. It is implausible, however, to think that plants are somehow more alive than non-rational animals or even humans. Hence, the first member in this series isn't the primary (core) way of being alive. There is no suggestion that there is a core way of being alive in Aristotle's definition of life.

In this passage in De Anima II.3, Aristotle sets out the various entailment relations and poses the question of what explains each one:

And what the cause is due to which they have this order must be investigated. For without the nutritive part there is no perceptual one, though the nutritive is separated from the perceptual one in the case of plants. Again, without touch none of the other sorts of perception is present, but touch is present without the others. For many animals have neither sight nor hearing nor perception of odors. And of the ones that can perceive, some have the part capable of causing movement with respect to place, whereas others do not. Finally, and most rarely, they have rational calculation and thought. For among the ones capable of passing away, those that have rational calculation also have all the rest, whereas of those that have each of the others, not all have rational calculation, on the contrary, some do not even have imagination, whereas others live by this alone.with respect to place, whereas others do not. Finally, and most ⁽³⁶⁾

Having set out these entailment relations, Aristotle now requires causal explanations for each one, but the type of causal explanations he will invoke for each need not be of one kind. Let's take each of these entailment relations and summarize Aristotle's explanation.

If x can think, then x has non-tactile perception ⁽³⁷⁾ (but not vice versa).

If x has non-tactile perception, then x can move itself (and vice versa).

If x has non-tactile perception and can move itself, then x has touch and appetite (but not vice versa).

If x has touch and appetite, then x can nourish itself (but not vice versa). ⁽³⁸⁾

First, why does the ability to think entail that something has non-tactile perception? Aristotle's explanation involves two steps: (i) acts of thinking depend on the exercise of imagination, and (ii) imagination is essentially a process brought about by acts of perception, which only an animal capable of perception can engage in. ⁽³⁹⁾

Second, why are non-tactile perception and the ability to move one's body from one place to another mutually entailing? Aristotle gives a final-causal explanation. The distance senses are necessary for an animal's ability to move itself to be of any actual benefit to it. It is because the animal can perceive potential food and mates at a distance that it is able to direct its motion towards these and away from dangers. If the animal lacked the distance senses, its ability to direct its own motion would be pointless.

Third, the possession of the distance senses entails the possession of touch but not vice versa insofar as touch is more necessary for the preservation of the animal than these others. For whether something is actually beneficial or harmful to the animal is determined by what it does to the animal when it's in contact with it. These benefits and harms are perceived as pleasures and pains by the animal, and hence it is the association of certain sights, sounds, and odors with tangible or tasteable pleasures or pains that make the distance senses useful. Without touch, none of the distance senses is actually useful for the survival of the animal.

Finally, nourishment is necessary for anything to be alive insofar as that thing is something which is generated at a certain time. Hence, the presence of nutrition is entailed just by the living thing's being something that came into existence at a certain time. Why is this? Because what is generated must (at least typically and as a species) have a time of growth, a prime at which it is at its mature size, and a period of decline until its death. Each of these can only occur in something which is being nourished where (i) during its growth period it takes in more nourishment than it needs to maintain itself, (ii) during its prime it takes in enough nourishment to maintain itself, and (iii) during its decline it takes in less nourishment to maintain itself.

How does Aristotle explain the failure of entailment in certain cases? For instance, why is it that an animal can have touch without having the other senses? Here, Aristotle appeals to an absence of hypothetical necessity. Is it possible for an animal to survive without being able to perceive its potential food from a distance? Yes, so long as it is able to be nourished by whatever is likely to come within its grasp. Hence, the survival of certain animals doesn't require that they have the distance senses, and hence there is no final-causal reason for them to have such senses. Couldn't there be a material-causal or efficient-causal reason for them to have such senses however? That seems possible (indeed, one might take the mole as a more limited instance of this, given its blindness). However, such reasons would only explain why there were some instances of animals with distance senses which served no purpose for their survival, and it wouldn't give an explanation for why all animals with touch need have the distance senses.