(Book 2, Lessons 1-4)

After the determination of the principles of natural things, namely subject, form and privation, Aristotle turns to the determination of the principles of natural science. What are they? In any science it is necessary to know its subject and the middle term through which it demonstrates. Consequently, the first thing to be treated in this respect is the subject of natural science.

3.1 The definition of nature

Since the subject-matter of natural science is mobile being, which is equivalent to the world of nature, it is necessary to begin by defining nature.

Among the things around us, we say that some are "natural", while others are "artificial", or else they are designated in some other way, such as they are the result of "chance".

We call animals "natural", also plants and elements. What is the common note in all these, that is implied in our calling them "natural"? It is that they have within themselves some principle of motion and rest, whether this motion is growth, or local motion, or qualitative change.

Artificial things do not have as such a principle of motion within themselves, but only that of the natural substances which compose them. Thus a car does not move of itself, but by reason of distinct, independent parts which compose it, reacting to the combustion of fuel.

This principle of motion in natural things may be either active or passive. Thus fire burns actively, while a body attracted by a magnet is moved passively. It is an intrinsic ability to act or to be acted upon. Gravitational motion also is passive, but comes from active principles:

For gravity in earth is not a principle ordained to its moving actively, but rather that it be moved, since just as other accidents follow upon substantial form, so also does place, and consequently being moved to a place, not however in such a way as for the natural form to be the mover, but the mover is the generator which gives such a form, upon which such a motion follows.

Since natural things differ from the non-natural by virtue of having their principle of motion within themselves, it is possible to arrive at the following definition:

Nature is the principle of motion and rest in that in which it is primarily and per se and not according to accident.

      "Principle": This principle whence motion originates either actively or passively is found in living things when one thing gives birth to or generates another. Hence the word "nature" is from the Latin nasci, natum, "to be born"; Greek comes from - "to sprout". Nature, as form, is sometimes a cause in the active sense of acting on another, sometimes in the passive sense of initiating self-motion, as in the case of gravity. Matter it is a cause in the passive sense of receiving motion, either from the form [natural motion] or from without [violent or para/supra-natural motion].

      "Of motion and rest": Just as nature designates the principle which causes something to move to a certain place, as in the case of gravity, so also nature is the principle of remaining at rest in its proper place when it reaches it.

      "In that in which it is": In distinction from artificial things, like machines, which do not have a nature of their own and only have the nature of their natural components, nature is in natural things.

      "Primarily": Thus in composite things, even though natural, one must distinguish between what is generic or remote and what is proper to the nature in question. For example, when an animal falls, it is not because of its nature as animal, but because of the generic nature of the matter out of which it is composed.

      "Per se and not according to accident": Thus when a doctor cures himself, the active principle of his cure is within himself, but only accidentally: The patient as patient has within himself only the ability to be cured, not the active ability to cure, as a doctor has. In all artificial things, the active principle is from without, as in the case of a house or a table.

The existence of nature is not demonstrated, but is self-evident. It is plain to the senses that there are many things which have the principle of their motion within themselves, i.e. are from nature. To try to prove the existence of nature is to try to prove the self-evident by something something that is less evident: the more known by the less known. However, what the nature of each thing is, or what is the principle of its motion, is not immediately clear and evident. Thus we can know that there is a principle of motion within something without knowing what it is.

3.2 The different senses of nature

3.2.1 Matter as nature

For the first natural philosophers, who had not yet arrived at the concept of prime matter, the basic matter of all things was some sensible body such as fire or air or water. Consequently, since all forms came to matter already in existence, they were in the order of accident, as with the forms of artificial things. The basic substantial elements might be either one or more than one. Empedocles, for example, proposed what became the traditional four: air, earth, fire and water.

Not only were these material elements the substance of things, but also they were perpetual and incorruptible, while the succeeding accidental forms they received came and went.

This opinion was true in that it considered matter as the subject and a constitutive part of natural things, but it was false in considering matter the whole substance. For them, there was no natural unit beyond the elements.

3.2.2 Form as nature

Actually a thing does not come into being until it receives a certain form. It is the form which gives it identity and actuality. We do not call something which is only potential by the name of that which is actual; we do not call a piece of lumber chair when it is potentially a chair, but only when it is actually a chair.

Matter is nature, and form is nature, yet the composite is not properly called nature, since it is the product of nature. That is because nature is a principle, and the composite is an effect or product of the principles. [Elsewhere in a secondary or loose sense, Thomas sometimes uses the word "nature" as equivalent to "essence", which is the composite of matter and form. Another equivalent word is "substance", which is the composite as the subject of accidents.]

Since the idenity of a thing comes more from its act than from its potency, the word nature applies more to the form than to the matter.

3.3 How natural science differs from mathematics

We have seen in Chapter 1 that while natural science abstracts from matter in its individuality, mathematics abstracts from the aspects of matter that are proper to the different senses, such as sound, colour, smell and hardness.

It is plain that posterior things are not comprised in the understanding of prior things, but conversely: whence the prior may be understood without the posterior, and not conversely. Thus it is evident that "animal" is prior to "man", and "man" is prior to "this man" {for man is in addition to animal, and this man is in addition to man. And because of this, "man" is not comprised in the understanding of "animal", nor "Socrates" in the understanding of "man"} whence animal may be understood without man, and man without Socrates and other individuals. And this is to abstract the universal from the particular.

Likewise among all the accidents which come to substance, first there comes to it quantity, and then the sensible qualities and actions and passions and the motions following upon sensible qualities. Thus quantity does not comprise in its understanding sensible qualities or passions or motions; nevertheless it does comprise in its understanding substance. Therefore quantity may be understood without matter subject to motion and sensible qualities, nevertheless not without substance. And therefore such quantities and whatever occurs to them are according to understanding abstracted from motion and sensible matter, but not from intelligible matter, as is stated in Metaphysics VII.

Since, then, they are so abstracted from motion according to understanding that they do not comprise in their understanding sensible matter subject to motion, the mathematician is therefore able to abstract them from sensible matter. And it makes no difference as to the truth of the consideration whether they be considered in this way or that. For although they are not abstracted as to their being, nevertheless the mathematicians, who abstract them according to understanding, do not lie, since they do not assert them to exist outside of sensible matter, for this would be a lie. But rather they consider them without considering sensible matter, which may be done without untruth --just as one may consider "whiteness" without "music", and truly so, even though they should co-exist in the same subject. But it would not be a true consideration if one should assert that "white" is not "musical".

By virtue of the failure to recognize that things which may be understood apart, i.e. abstracted, do not necessarily exist apart, the Platonic school conceived of both types of abstraction as actual categories of being. According to Plato, in addition to individual men, there exists a universal man, called an "idea"; likewise, in addition to sensible bodies, there exist mathematical bodies devoid of any sensible qualities.

Although the concept of material body does not involve singular material characteristics, it does nevertheless involve universal material characteristics, and as such stands for the individual material body --but there is no individual body corresponding to a universal body conceived as devoid of sensible characteristics.

The mathematician, whose subject matter is the odd and even, straight and curved, number, line and figure etc., defines without motion and matter. Not so with the natural scientist: his definitions, such as those of flesh, bone, man etc., comprise sensible matter.

3.4 The mixed sciences

The mixed, or intermediate, sciences are those which take their principles from purely mathematical sciences, and apply them to sensible matter. For example, perspective (in painting or architecture) applies to visual lines what geometry demonstrates concerning abstract lines; music applies to sounds what arithmetic considers concerning numerical proportions; astronomy applies both arithmetic and geometry to the heavens.

These sciences, even though their principles are derived from mathematics, nevertheless are considered more natural than mathematical. Why? The reason is because things have their determination from their term, and the term of these mixed sciences is natural things.

In this respect they are the converse of the purely mathematical sciences, for these begin from sensible matter and terminate in the abstract. The mixed sciences begin from the abstract and terminate in sensible matter. Because of this application on the part of the mixed sciences, it is plain that the purely mathematical sciences must abstract from sensible matter.

Since, however the mixed sciences are not purely natural, they will demonstrate their conclusions by another middle term than the natural sciences. Thus, whereas the natural scientist will demonstrate the sphericity of the earth from the properties of the matter composing it, i.e. by virtue of all its parts tending to the centre, the astronomer will do so, for example, from the shape of a lunar eclipse, or from the fact that the same stars are not seen from every part of the earth. [Most modern physics is "mixed science"].

3.5 Natural science considers both matter and form

Since both matter and form are nature, as principles of motion and rest, natural science treats of both. This is in contrast to the first natural philosophers who thought of nature primarily in terms of matter, [and to the tendency of some modern scientists to restrict their consideration to the quantifiable or measurable aspect of things.]

That it is necessary to consider form equally with matter may be seen from all the arts, where both are considered: the builder considers his materials in view of the form of the building; the doctor considers medicine in view of the health he wishes to attain. Since the arts imitate nature, the same importance of form must be true of nature.

Another reason why natural science considers both form and matter is the fact that form is the end of matter. In effect, the final form intended dictates what will be done to the matter. Thus in the arts, the one who designs the form of a ship specifies what the matter shall be. The form of the ship induced into the matter is the end of the construction of the ship; ultimately the use of the ship determines the form, and the form determines the matter.

3.6 The limit of natural science

Natural science considers "nature", "form", "cause" etc. only in so far as these exist in matter. To consider them absolutely, i.e. indifferently whether they exist in matter or are immaterial, belongs to metaphysics or "first philosophy". Aristotle uses the comparison of a doctor and sinews: the doctor does not consider the sinew as such {since this belongs to the natural scientist} but only in so far as it pertains to health; so the natural scientist considers the forms of things only as they are in matter.

The limit of the consideration of natural science is, therefore, those forms which in a certain way are separated from matter, but still have their being in matter, namely, rational souls. These are separated in that the intellective power is not the act of any corporeal organ, but are in matter in that they give natural being to the body they inform.

The consideration of forms totally separated from matter, however, and even of the rational soul in the state of separation from the body, pertains to first philosophy.


Galileo, dialogues concerning two new sciences:

I desire, before passing on to any other subject, to call your attention to the fact that these forces, resistances, moments, figures etc. may be considered either in the abstract, dissociated from matter, or in the concrete, associated with matter. Hence the properties which belong to figures that are merely geometrical and non-material must be modified when we fill these figures with matter and therefore give them weight.

William Harvey, Anatomical exercises on the generation of animals:

Although there is but one road to science, that to wit, in which we proceed from things more known to things less known, from matters more manifest to matters more obscure: and universals are principally known to us, science springing by reasonings from universals to particulars: still the comprehension of universals by the understanding is based upon the perception of individual things by the senses.

Arthur Eddington, The nature of the physical world:

Let us then examine the kind of knowledge which is handled by exact science. If we search the examination papers in physics and natural philosophy for the more intelligible questions, we may come across one beginning something like this: "An elephant slides down a grassy hill-side." The experienced candidate knows that he need not pay much attention to this; it is only put in to give an impression of realism. He reads on: "The mass of the elephant is two tons." Now we are getting down to business; the elephant fades out of the problem and a mass of two tons takes its place. What exactly is this two tons, the real subject-matter of the problem? It refers to some property or condition which we vaguely describe as "ponderosidy" occurring in a particular region of the external world. But we shall not get much farther that way; the nature of the external world is inscrutable, and we shall only plunge into a quagmire of indescribables. Never mind what two tons refers to; what is it? How has it actually entered in so definite a way into our experience? Two tons is the reading of the pointer when the elephant is placed on the weighing machine. Let us pass on. "The slope of the hill is 60o." Now the hill-side fades out of the problem and an angle of 60o takes its place. What is 60o? There is no need to struggle with mystical conceptions of direction; 60o is the reading of a plumb-line against the divisions of a protractor. Similarly for other data of the problem. The softly yielding turf on which the elephant slid is replaced by a coefficient of friction, which, though perhaps not directly a pointer reading, is of a kindred nature. No doubt there are more round-about ways used in practice for determining the weights of elephants and the slopes of hills, but these are justified because it is known that they give the same results as direct pointer readings.

E.J. Tarbuck & F.K. Lutgens, Earth Science (1982), pp. 3-4, 440:

Minerals, like all matter, are made up of elements. At present, more than 100 elements are known, a dozen of which have been produced only in the laboratory. Some minerals like gold are made entirely of one element, but most are a combination of two or more elements joined chemically to form a compound.

In order to better understand how elements combine into compounds, we will first consider the smallest part of matter that still retains the characteristics of an element {the atom} because it is this extremely small particle that does the combining. Each atom has a central region, called the nucleus, which contains very dense positively charged protons and neutral particles called neutrons. The number of protons determines the atomic number and name of the element. For example, all atoms with six protons are carbon atoms, all those with eight protons are oxygen atoms, and so forth. Orbiting the nucleus are negatively charged particles called electrons, which are equal in number to the protons. As a result, the opposite charges of an atom balance each other.

When an atom combines chemically, it either gains, loses, or shares electrons with another atom....

In ordinary atoms the electrons circle the nucleus at distances proportinal to the orbits of the planets around the sun. Thus, like our solar system, atoms are made up mostly of empty space.