What Natural Bodies Are

A Theory of the Existence of Natural Bodies in terms of Forms

Some things we find in the world, and other things we make. The things we find do not depend on us for their existence, unlike the tables, chairs, and other things we make. We can say that the things we find are "natural bodies," and we can ask whether they really exist on their own and how they manage to exist like this if in fact they do.

The noun nature derives from the Latin natura, which was a standard translation of the Greek noun φύσις.

The Latin noun physica transliterates the substantive use (the use as a noun) of the Greek adjective φυσική.

In the phrase 'inquiry into nature,' the Greek for 'inquiry' is ἱστορία. This Greek noun transliterates as historia.

The title of Herodotus's Histories is Ἱστορίαι.

"This is the display of the inquiry (ἱστορίης) of Herodotus of Halicarnassus, so that things done by man not be forgotten in time, and that great and marvelous deeds, some displayed by the Hellenes, some by the barbarians, not lose their glory, including among others what was the cause of their waging war on each other" (Histories I.1).

The term 'natural history' preserves some of the ancient sense of 'inquiry into nature' (ἱστορία περὶ φύσεως). This is due in part to the importance in the history of science of Aristotle's Περὶ Τὰ Ζῷα Ἱστορίαι ("Inquiries about Animals"), which is commonly called by its Latin title Historia Animālium or English translation History of Animals.


Book II of the Physics:

Chapter 1. The nature in a thing
Chapter 2. The physicist
Chapter 3. Causes.
Chapter 4. Outcomes by luck and by accident
Chapter 5. Luck
Chapter 6. Accident
Chapter 7. Summary thus far
Chapter 8. The for something
Chapter 9. Necessity


"Of things that are, some are by nature, some because of other causes. By nature are the animals and their parts, and the plants and the simple bodies (earth, fire, air, water)—for we say these and the like exist by nature. They seem distinguishable from things not united by nature. Each of the beings that exist by nature has within itself a starting-point of change and staying unchanged" (Physics II.1.192b).

"For nature is a starting-point and cause of being moved and of being at rest in that to which it belongs primarily according to itself and not accidentally" (Physics II.1.192b).

"Nature is said to be the shape and form according to the account [of what the object is]" (Physics II.1.193a).

"What is potentially flesh or bone has not yet its own nature, and does not exist by nature, until it receives the form according to the account, which we name in defining what flesh or bone is. ... So the nature of things having a starting point of motion is the shape or form, which is not separable except according to the account . ... And this is more [what] nature [is] than the material [is what nature is]" (Physics II.1.193b).

"separate" (χωριστὸν)

"To what point should the physicist know the form and the what it is (τὸ εἶδος καὶ τὸ τί ἐστιν)? ... [To the point of knowing] what is separable in form but in matter (χωριστὰ μὲν εἴδει, ἐν ὕλῃ δέ) ... What is separable [without qualification], and how things are with it, is the work of first philosophy to determine" (Physics II.2.194b).
Aristotle thinks that the natural bodies exist by having a "nature" (φύσις).

This nature is the "starting point of change and staying unchanged" in the body. It unites the differences in a natural body at different times in its history so that these differences are changes in the history of one natural body that persists through time.

The details of what Aristotle has in mind are challenging to see, but it seems clear that he is trying to explain something we commonly think: that human beings and other living natural bodies we find in the world are objects that can persist through change. The philosophical challenge is explain how this is true and hence why these things are not really just collections of more basic things, and Aristotle proposes that it is because they have a nature.

For his explanation to be plausible, as he himself realizes, we need to know what a nature is.

Natures are Forms in Matter



Theseus slays the Minotaur. Theseus slays the minotaur

In the Phaedo, Echecrates asks why Socrates spent so much time in jail after his conviction. Phaedo answers that the execution was delayed because the ship of Theseus had not returned from Delos. Athens had to be kept pure until the ship returned, so no executions were carried out.

Delos (Δήλος) is an island in the Aegean Sea.
To understand what Aristotle thinks a nature is, it helps to think about the prior history.

The Presocratic tradition after Parmenides eliminates natural bodies. Democritus conceives of them as the product of a "bastard" way of thinking about atoms in the void. In this Presocratic tradition, what we think of as natural bodies are illusions our minds create. They are atoms located close together in the void that appear to us as objects that can persist through change.

Just how Plato understands natural bodies in the Timaeus is not completely clear, but he seems to think that forms are the reality of natural bodies. Aristotle accepts this general line of thought. He thinks that natures are forms, but he also understands the existence of these forms in a new way. He does not think that the forms of natural bodies are "separate," as Plato seems to have thought. He says they are "in matter" and only "separate in account."

We need to know what Aristotle means by "in matter" and "separate in account."

The Ship of Thesesus


"The ship on which Theseus sailed with the youths and returned in safety, the thirty-oared galley, was preserved by the Athenians down to the time of Demetrius Phalereus [fourth to third century BCE]. They took away the old timbers from time to time, and put new and sound ones in their places, so that the vessel became a standing illustration for the philosophers in the mooted question of growth, some declaring that the vessel remained the same, others that it was not the same" (Plutarch, Life of Theseus 23 (in Parallel Lives (a set of biographies of famous men))).
We can use an ancient thought experiment to help us.

The subject of this thought experiment is the ship of Theseus, and the question is what makes the ship be one object that has persisted so long through its many repairs.

(The ship was thought to be ancient in Socrates' day and to have been repaired many times.)

If we ask what makes the ship that was repaired and the ship that first came into existence be the same ship, its material does not seem to be the answer. The boards and other parts were replaced as the ship was repaired, but it is implausible to think that repairing the ship caused the unrepaired one to go out of existence and a new one to come into existence. It seems instead that what makes the unrepaired and repaired ship be the same ship is the organization of the material. This organization does not change in the course of the repairs to the ship. It is what persists and makes the ship of Theseus exist as one ship that was repaired over time.

Aristotle has this sort of view for natural bodies. He thinks that the organization unites the material in a natural body so that it is one object that can persists through changes (such as the "repair" the occurs in living bodies when they recover from illness).

Aristotle's Theory of Forms

The ship of Theseus example provides some insight into how the forms of natural bodies are

• "in matter" and are only "separate from the matter in account"

Because the forms of natural bodies are each the organization of some "matter," they cannot exist apart from matter. We can, though, think about this organization without thinking about the matter it organizes, and this makes the forms separate from matter "in account."

To understand this a little more clearly, consider the example of the ship again.

The form of the ship is the particular organization of boards and other materials so that there exists a ship of a certain kind. Removing this organization from these boards and other materials causes the organization to go out of existence because it destroys the ship by breaking it apart. So, in the absence of this organization, there is no ship of Theseus. There is only a pile of boards and other materials we might use to try to put the ship back together.

There is a way, though, Aristotle thinks, that the form is separate from the matter.

The ship of Thesus is a galley, and a ship of this form is a ship that functions in a certain way. It is a ship propelled primarily by oars, has a shallow draft, and has low freeboard. This description makes no mention of the pieces of wood and other things we use to build galleys. This, Aristotle thinks, makes the form of the ship separate in account from the matter.

When Aristotle says that the form is "in matter and separate from it in account," he is giving what he takes to be the truth in Plato's conception the existence the forms of natural bodies. Aristotle thinks Plato was right that the forms of natural bodies are separate, but he was wrong to think they are separate without qualification. As Aristotle understands the forms of natural bodies, they exist in matter and can exist separately from it only "in account."

We need to consider in more detail this mode of existence Aristotle thinks the form possess, but now we can state a consequence of his view. For the natural bodies of some kind,

• the forms are numerically distinct but identical in account

Each form is a numerically distinct organization of some materials. For a given kind of natural body, these organizations make collections of materials be or exist as bodies of the kind.

"Your matter and form and moving cause are different from mine, but they are the same in their general account [because we are both human beings]" (Metaphysics XII.5.1071a).




Perseus Digital Library

Aristotle, Metaphysics
A Dictionary of Greek and Roman Antiquities: THEO´RIS (θεωρίς), THEO´RI (θεωροί)

Henry George Liddell, Robert Scott, A Greek-English Lexicon

θεωρία, theōria, noun, "a sending of θεωροί or state-ambassadors to the oracles or games, or, collectively, the θεωροί themselves.
θεωρίς, theōris, noun, "sacred ship, which carried the θεωροί to their destination"
θεωρός, theōros, noun, "envoy," ("spectator" in the sense of "overseer")

The θεωροί ("overseers") were sacred ambassadors sent on special missions (θεωρίαι) to perform some religious duty for the state, to consult an oracle, or to represent the state at some religious festival. In Athens, it seems that three ships carried the θεωροί: the Delia (Δηλία), the Salaminia (Σαλαμινία), and the Paralus (Πάραλος).) The Greeks thought that the Δηλία was very old and that it had once taken Theseus to Crete.

Arizona State University Library. Loeb Classical Library.
Aristotle, Physics




"Aristotle thinks that the capacity of an object to behave in this characteristic way [that characterizes it as a member of a natural kind] depends on its organization, structure, and disposition, indeed, he thinks that it is just this disposition or organization that enables the object to behave the way it does. Now, for Aristotle, the form is this disposition or organization, while the matter is what is thus disposed or organized. How could the form, so construed, satisfy the requirements laid down for being a substance? An important requirement was that the substance was to explain why, despite all the changes an object had undergone, it still is the same object. How the form could satisfy this requirement, we can see from the ancient example, expanded by Hobbes, of Theseus's ships, Theoris, which for centuries has been sent to Delos on an annual pilgrimage and whose return Socrates, in the Phaedo, must await before he may drink the poison. Over the years, the ship is repaired, plank by plank, always, however, according to the original plan. Now, let us suppose there is a shipwright who keeps the old planks. After all the old planks have been replaced in Theoris, he puts them together again according to the original plan and thus has a second ship. It seems obvious to me that this ship, even though it is constructed from all the old planks and according to the original plan, is not the old ship, Theoris, but a new ship; the ship constructed from the new planks is, in fact, the old ship. ... What makes for the identity of the repaired ship with the original ship is obviously a certain continuity. This is not the continuity of matter, or of properties, but the continuity of the organization of changing matter, an organization which enables the object to function as a ship, to exhibit the behavior of a ship" (Michael Frede, "Individuals in Aristotle," 66. Essays in Ancient Philosophy, 49-71).




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