IV. NATURE.
THE definition of the word nature, as used by the Scholastic doctors, is taken from the second book of Aristotle's Physics. "NATURE," he says, "IS THE FIRST PRINCIPLE OF MOTION AND OF REST, per se, not per accidens." To understand this definition, we must remember that a substance may be moved by an intrinsic or an extrinsic principle. If it is moved by an extrinsic principle, the motion is forced. If it is moved by an intrinsic priuciple, the motion is natural. When one billiard ball is sent at another, the impulse is extrinsic and the motion forced; but when two drops of mercury, placed near each other, approach and meet, their motion is from an intrinsic principle, and is natural.
This principle is called nature. But, besides being a principle of motion, it must also be a principle of rest; for nature inclines things to move, not for the sake of moving, but because it tends to a scope, an end, a bonum, which being attained, rest supervenes. Without some obstacle or attraction the motion given to the billiard ball would be perpetual, because it was not directed to a term fixed; whereas the drop of mercury will rest as soon as it has touched the other drop of mercury, to which it is brought by the principle that causes its motion. In this definition of nature, as the principle of motion and of rest, the words prima and per se, contradistinctive to per accidens, are put to distinguish the principle whence the motion proceeds (which is nature) from any modification or accidental affection of it, without which it would either not be set in motion towards the object, or inclined with a different intensity -- a very valuable distinction in Catholic theology for distinguishing nature from grace and the natural from the supernatural. And this much will suffice about the physical meaning of the word nature.
Let us now see what constitutes it in a corporeal substance. In this there is matter and form. Matter alone would not be sufficient, because matter (materia prima) remains the same under all substantial forms, and therefore, were it per se a principle of operation, it would always operate in one way. But that does not happen. Oxygen, for instance, and water are different in their operation, though all the matter (materia prima) that is in one is likewise in the other. Does the substantial form, then, constitute the nature of corporeal substance? If we consider the matter as per se inert, and the form as the only other thing in corporeal substances, we must say that the form is the principle of operation or of motion. But would it be so, if it were separated from the matter? No, for it could not exist at all. And therefore, to be the principle of operation and of motion, it must be united with matter. The word nature does indeed principally apply to it, but not with propriety as prescinding from the matter which it informs. Rightly therefore did Aristotle say in the second book of Physics, that as copper, if we prescind from its figure, cannot be called art, neither can matter (materia prima), when considered apart from the substantial form which determines it to a certain species, be called the nature of a thing; but that name should rather be given to the form.{1} "As that may be called art," says St. Thomas, "which belongs to anything that is according to art and artificial, so may that be called nature which belongs to anything that is according to nature and natural. But that which is only in potentia to be made by man's art cannot be said to have anything of art in it, because it has not as yet the nature of (for instance) a bed. Therefore in natural things that which is flesh and bone in potentia has not the nature of flesh and bone till it has received the form, according to which is the definite nature of the thing, and through which we know what flesh is and what bone is. There is no nature in it till it has its form; and therefore the form is in a way the nature of natural things that have in them the principle of motion."{2}
From this we can easily understand why the Scholastics called matter and form incomplete substances, and the compositum a complete substance. For that being which naturally is of itself is a substance; and matter cannot be of itself without the form, nor can a material form (for of that we are here speaking) be without matter. True it is that neither the one nor the other can be called an accident, because each has its own entity; but when disjoined they are wanting in what is required for the definition of substance, and therefore are incomplete. This they acquire when conjoined; and then they are a complete substance. "We call a substance physically incomplete," says Suarez in his Metaphysics, "that by its entity has not in itself what is required for the nature of substance taken generically; and that which has it we call a complete substance. This we express by means of negation, saying that a substance is physically complete which is not ordained per se to perfect or to constitute with another substance another being."{3}
And Cicero speaks likewise in these words De natura autem ita dicebant (Aristotle and his followers) ut eam dividerent in res duas, ut altera esset efficiens, altera, quasi huic se praebens, ea quae efficeretur aliquid. In eo quod efficeret, vim esse censebant in eo autem quod efficeretur materiam quamdam: in utroque tamen utrumque. Neque enim materiam ipsam cohaerere potuisse, si nulla vi contineretur, neque vim sine aliqua materia. Nihil est enim quod non alicubi esse cogatur. Sed quod ex utroque, id jam corpus nominabant. {4}
Here it is evident that, since what he calls vim cannot be without matter, nor matter without form, each is an incomplete substance, and the body alone, composed of the two, is a complete substance.
Moreover, if the substantial form principally constitutes the nature, clear it is that plurality of such forms brings plurality of natures. Hence a body can never be considered as one nature, if it be nothing more than an aggregate of atoms, each furnished with its own substantial form. Even a man could not be said to have one complete nature, if there were in him more than one substantial form; for with each form he would have a complete nature. And therefore St. Thomas argues thus: "One thing results from many things, firstly according to order alone, as a city is made of many houses, or an army is made of many soldiers; and secondly by order and composition, as a house is made by contact and junction of parts. But these two ways do not suffice to constitute out of many things one nature; and therefore those things that have a common form in order or in composition are not natural things, whose unity can be called the unity of nature."{5} And therefore, if we suppose a body as formed by mere aggregation of atoms or molecules, we cannot call it one in nature, though all its parts concur in one operation as to the term. One nature implies not only one term, but also one principle of operation, which cannot be where many operating things have a divided being. "It is impossible," says St. Thomas, "that there can be one operation in things which differ in their being. I say one, not on the part of that in which the action terminates, but as it comes from the agent; for many men dragging a boat do one work as to the thing done, which is one, but on the part of the men who drag it the actions are many, because the impulses that move the boat are many."{6} Nor can it be said that we may suppose each atom to be an incomplete nature which is completed by the aggregation of many atoms; for from what we have shown about the two natures (the complete and the incomplete), it is evident that each atom would really be a complete substance and nature, and therefore that the whole would only have a collective unity.
We say this because it is important to make the meaning of the word nature quite clear, seeing that it is not always in these times as clear as in the days of old. Any one may see how necessary it is to be clear about this. If there were no other reason, the Catholic definitions concerning man, and above all, concerning the Divine Word Incarnate, ought to be sufficient.
{1} Phys., Lib. ii. Cap. i.
{2} In II. Phys., lect. 2.
{3} Disp., 33, Sect. I. n. 3.
{4} Acad., i. 6.
{5} Contra Gent., iv. 35.
{6} Contra Gent., ii. 57.