The Natures of Natural Bodies
Natural bodies, according to Aristotle, have "natures." In conceiving of natural bodies in this way, Aristotle is part of an intellectual tradition that seems to begin with Thales and the Milesian inquirers into nature. Just what these inquirers into nature thought is hard to know, as almost nothing of what they wrote has survived, but it appears that they thought that reality has a nature (air, in the case of Anaximenes) and that ordinary phenomena (such as drops of water forming in the sky and falling to the ground in rain) occur in terms of changes in this nature.
(The word nature is derived from the Latin natura, which was a standard translation of the Greek word φύσις (physis). The Greek for 'inquiry' in the phrase 'inquiry into nature' is ἱστορία. It transliterates as historia. (The title of Herodotus' Histories is Ἱστορίαι.) The term 'natural history' retains some of the ancient sense of "inquiry into nature." In large measure, this is due to the importance of Aristotle's Περὶ Τὰ Ζῷα Ἱστορίαι ("Inquiries about Animals"), which is commonly called by its Latin title (Historia Animālium) and English translation (History of Animals), in the history of science.)
Aristotle thinks that the nature of a natural body is the "starting point of change and staying unchanged." The presence of the nature is the unifying factor in the differences in the history of a natural body according to which these differences in its history are changes in one object. In the absence of the nature, there would only be a sequence of different heaps of material.
"Of things that exist, some exist by nature (φύσει), some from other causes. By nature the animals and their parts exist, and the plants and the simple bodies (earth, fire, air, water)—for we say that these and the like exist by nature. All the things mentioned present a feature in which they differ from things which are not constituted by nature. Each of them has within itself a starting-point of change and staying unchanged (ἀρχὴν ἔχει κινήσεως καὶ στάσεως).... For nature is a cause of being moved and of being at rest in that to which it belongs primarily, in virtue of itself..." (Physics II.192b).
"In one way nature (φύσις) is said to be the ultimately underlying material (ὕλη) of all things that have in themselves the principle of movement and change. In another way, nature is said to be the shape or form according to the account (ἡ μορφὴ καὶ τὸ εἶδος τὸ κατὰ τὸν λόγον). ... 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. ... [N]ature [said in this way] is the shape or form (not separable except according to the account (οὐ χωριστὸν ὂν ἀλλ' ἢ κατὰ τὸν λόγον) of things in them which is a starting point of motion. And this is more [what] nature [is] than the material (ὕλης) [is]" (Physics II.1.193a-b).
"To what point should the physicist know the form and the essence (τὸ εἶδος καὶ τὸ τί ἐστιν)? ... [To] that what is separable in form but in matter (ἅ ἐστι χωριστὰ μὲν εἴδει, ἐν ὕλῃ δέ) ... What is separable, and how things are with it, is a question for first philosophy to determine" (πῶς δ᾿ ἔχει τὸ χωριστὸν καὶ τί ἐστι, φιλοσοφίας τῆς πρώτης διορίσαι ἔργον)" (Physics II.2.194b).
Aristotle thinks that a natural body is something more than the material that constitutes it. A natural body has a "nature" (φύσις). It has a certain unity or oneness. Contrary to what Democritus and Leucippus seem to have thought, a natural body is not just an arrangement of atoms in the void that happens to appear as an object to human beings.
Natures are Forms in Matter
To understand what a "nature" (φύσις) is, Aristotle considers the two philosophical traditions that come before him. The Presocratic tradition after Parmenides eliminates ordinary natural bodies in favor of an ontology in which the only objects are those that constitute the nature of reality. Democritus and Leucippus, e.g., seem to say that atoms and void are the thing things that exist. The Platonic tradition is a little harder to comprehend, but the leading idea is that forms are the fundamental objects and reality that underlie the world of experience.
Theseus slays the Minotaur
Aristotle sides with the Platonist tradition against the Presocratic tradition.
As a Platonist, Aristotle thinks that natural bodies have to be understood in terms of forms. He thinks that in the absence of form, there is nothing more than a heap of material. This heap is only "potentially" a natural body. Aristotle thinks that the form is the organization of the material such that the material constitutes a natural body. These forms, however, do not exist in the way that Plato supposed. Aristotle is a Platonist, but he is also the first great Platonic critic. Aristotle thinks that the forms of natural bodies are in matter and separate only according to the account.
An example helps to clarify Aristotle's conception of natural bodies.
Consider the ancient ship of Theseus. (It is mentioned in the Phaedo.) This ship was thought to be old by the time of Socrates' death and to have been repaired many times. So the question arises as to what makes the ship the same over time. The answer does not seem to be its material. The boards and other parts of the ship were replaced many times as the ship was repaired, but it seems intuitively implausible to think that repairing the ship caused the original one to go out of existence and caused a new one to come into existence. So the sameness of the material does not seem to be what makes the ship the same over time. What, then, makes the ship the same? Its identity appears to consist in the organization of the material that constitutes the ship. This organization does not change in the course of repairs. The organization appears to be what persists and makes the material be the ship of Theseus.
The ship of Theseus is an artifact, not a natural body, but the underlying conception of the identity of the ship seems to apply to the identity of natural bodies more generally. Their identity does not seem to consist in the material that constitutes them. Instead, it seems to consist in the way this material is organized so that there exists a natural body with various features. What is this organization? Aristotle is a Platonist. He thinks that his organization is a form, that this form is in matter, that a form in matter is what a natural body is over and above the material that constitutes it, and that forms in matter are the reality of natural bodies.
(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 Crete. The city had to be kept pure until the ship returned, so no executions were carried out during that time. Plutarch (a Platonist philosopher who lived c. 45 – c. 120) reports that the Athenians preserved the ship from the time of the Theseus down to the time of the orator, Demetrius of Phalerum (350 – 280 BC).)
Aristotle's Theory of Forms
Aristotle thought that the forms of natural bodies are in matter and separate only in account. By so describing the forms of natural bodies, Aristotle is making a point in ontology. He is explaining how the forms of natural bodies exist. They are separate from the matter, but they are separate only with qualification. The qualification is "only in account."
Consider again the ship of Theseus. What would happen if one tried to separate the organization from the material? The answer, it seems, is that the ship would be destroyed. Without the organization, there is no ship. There is only a heap of material. The organization can be described abstractly, but to exist it needs to be in matter (the boards and so on) from which ships are built.
How is the form separate in account? The idea is difficult, but Aristotle may have something like the following in mind.
Suppose that triangles are natural bodies. The form of a triangle is what makes lines be a triangle. The lines must have a certain organization. They must be organized such that there is a triangle. One way to do this is to organize the material so that there are interior angles that sum to two right angles. Having interior angles that sum to two right angles is common to numerically distinct triangles. This organization is specified in the account of what a triangle is. A triangle is a figure whose interior angles sum to two right angles, but this account contains no mention of the material that constitutes the lines. In this way, the form may be described as separate from the matter in account. The form cannot exist apart from a given set of lines. Any attempt to pull the organization out of a triangle destroys the organization so that only a heap remains. The form is separate in a qualified way. It is separate in account. It is not separate from matter.
To understand Aristotle's conception of forms, it is helpful to contrast it with the one Plato sketches in the Timaeus.
Timaeus has "made knowledge of the nature of the whole his chief object" (27a). He sets out a conception of natural bodies in terms of the forms and something he calls the "receptacle." The discussion is difficult to understand in detail, but the idea is that natural bodies are a product of these two more basic parts of the ontology: the receptacle and the forms. In time, parts of the receptacle become like the forms fire, air, earth, and water. As a result, portions of fire, air, water, and earth are "born." The non-elemental bodies, in turn, are compounds out of these elements.
"Not only does the receptacle always receive all things, it has never in any way taken on any characteristic similar to any of the things that enter it. These are the things that make it appear different at different times. The things that enter and leave it are imitations of those things that always are, imprinted after their likeness in a marvelous way hard to describe. ... We must keep in mind types of things: that which comes to be, that in which it comes to be, and that after which the thing coming to be is modeled and which is the source of its coming to be. It is appropriate to compare the receiving thing to a mother, the source to a father, and the nature between them to their offspring" (Timaeus 50b-d).
Whereas Timaeus seems to oppose forms to bodies as things that exist completely separately from the bodies, Aristotle conceives of the forms of natural bodies as particular objects that exist in the matter that constitutes natural bodies. These forms are numerically distinct but identical in kind or species. Each is the organization of different material and hence is numerically distinct, but the organization itself is the same in each case. The organization is specified in answer to the question of what the species is. This organization of material is separate in account only.