Natural science seeks the causes of motion. These causes are found in the nature of things and in outside factors. One baffling question is whether everything that happens has a cause, since some things happen by chance, which seems to be the absence of any rational explanation or cause.

4.1 What is chance? (Book 2, Lessons 7-10)

4.1.1 Opinions on the subject

There are two opposite extremes among opinions regarding chance. The first is to deny chance altogether and attribute everything to determined causes. Thus if a man went to market to buy a suit and there met a debtor who paid him an old debt, this encounter is attributed to good luck --yet it had a determinate cause: the deliberate intention to buy a suit, which was also the cause of finding the debtor. Thus chance as a cause is fictitious.

The opposite opinion is that everything happens by chance: Aristotle states that certain men stated chance to be the cause of the heavens and of all the parts of the world. And they said that the revolution of the world and all the consequent seasons and happenings on this earth are from chance. Thus Democritus held that from the concurrence of atoms movable of themselves the heavens and the whole world were constituted by chance.

A third opinion makes chance a divine reason which we cannot understand, something like divine providence. But the arrangement of things by God cannot be called chance, since such events are reasoned and ordered.

4.1.2 Distinctions needed for defining chance

Some things happen always or regularly in the same way, while other things, like having six fingers, are exceptional. Chance is something that is exceptional to the rule; since such exceptions occur, it is not right to say that everything is determined.

Regularity is a characteristic of things that happen by nature, since nature always acts in the same way unless impeded. Human behaviour, coming from free, will is not necessarily regular, but the will, like nature, acts for a purpose, unlike what happens by chance. Chance results are unintended and unforeseen in the case of volitional acts, and exceptional in the case of natural causality; both the will and nature are accidental causes of what happens by chance.

4.1.3 Definition of chance

Chance may now be defined as: "the accidental cause of exceptional or unintended results of choice or nature acting for another purpose". Thomas uses "chance" as a generic term, and "fortune" (or "luck") for unintended results of human choice.

Chance and fortune are termed good or bad according to their effect on people. Also, just as chance applies to something that happens without being intended, it can likewise be applied to what does not happen, i.e. to missing what was intended, as when we say someone worked in vain.

To what genus of cause is chance reduced? Since it pertains to what happens by nature or by intellect (and will), and these are causes "whence comes the beginning of motion", chance is reducible to the efficient cause.

4.2 Natural science demonstrates through all four causes (Book 2, Lesson 11)

Chapter 1 noted how all four causes apply to natural science, and Chapter 2 defined each of these causes. As for the material cause, a scientist may demonstrate that because something is composed of contraries it will necessarily corrupt. As for the efficient cause, a scientist may demonstrate that as the sun's position moves to the north the rainy season comes, but as it goes back south the dry season follows. But both material and efficient causes can be impeded from having their normal effects, and the presence of these causes does not necessarily demand that their effects follow.

The formal cause is used in a hypothetical demonstration stating: "If such-and-such a thing is to be produced, then such-and-such material is necessarily required." As for final cause, a natural scientist may demonstrate that something is so because that is more fitting, for example, "The front teeth are sharp because they are better thus for biting food, and nature does that which is better." Nevertheless this does not mean that nature always does what is better absolutely speaking, but as befits the substance or nature of each thing --otherwise it would give every animal a rational soul, since that is better than an irrational soul.

4.3 Nature acts for an end (Book 2, Lesson 12)

Nature is among the number of causes which act for the sake of something. And this is important with reference to the problem of providence. For things which do not know the end do not tend toward the end unless they are directed by one who does know, as the arrow is directed by the archer. Hence if nature acts for an end, it is necessary that it be ordered by someone who is intelligent. This is the work of providence.

Those who attack final causality reject the supposition that nature is always seen to do what is best and most fitting, e.g. that a foot is made the way it is for the sake of walking. Arguing against this they would say that the good or utility which comes about from an operation of nature simply happens (by chance), while the process itself is determined by the necessity of matter. Thus it is the necessity of matter whereby the sun's heat causes water to evaporate and become clouds, which later cool, condense and fall as rain. The rain may happen to fall on crops and help them grow, but it may equally well happen to fall on ripe grain and spoil it.

They argue the same way when speaking of the parts of animals: Front teeth were not made sharp and back teeth broad for the convenience of eating, but they simply happened that way and the utility followed.

To the objection that such usefulness follows always or for the most part, and as such must be intended by nature, they answer that in the beginning many non-viable forms came and went and only the fit survived.

They state that in the foundation of the world the four elements came together to constitute natural things, and there were many and various dispositions of natural things. Now wherever everything came together in just the way that is suitable for some utility, as if they had been made for the sake of this, such things alone were preserved --since they had a disposition apt for preservation not from any agent intending an end, but from that which is "per se in vain", i.e. from chance. Whatever things did not have such a disposition were destroyed and do not exist today. Thus Empedocles stated that in the beginning there were certain beings generated which were part-oxen and part-men.

After stating this position (the essentials of later Darwinism) Aristotle gives several arguments against "natural selection by chance":

      What happens always or for the most part (regularly) is not from chance.

      Nature always acts as though for and end. This is clear from the similarity between nature and art; just as art proceeds towards a definite end with determinate means, so does nature. Thus medicine tries to imitate natural processes.

      The purposiveness of nature is most evident in animal behaviour, such as the activity of spiders and bees, which appear so intelligent. Yet they do not act from intelligence, but through nature, as is evident from the fact that they always act in the same way. For every swallow makes its nest in a similar way, and every spider makes its web in a similar way --which would not be the case if they were acting from intellect and art, for every builder does not make a house in a similar way, since the artisan is able to decide on the form of the artifact and can vary it.

      The form of a new life is the end of generation, and the nature of an end is that other things be done for the sake of it. Therefore the process of generation is for the sake of an end.

In reply to objections against nature's acting for an end, Aristotle declares:

      The "sins of nature", such as monstrosities, are not evidence that nature is not purposeful, because (1) like art, the recognition of errors supposes that there is a right way for achieving a certain end. (2) The determined principles and determined order of development, as in the case of an embryo, indicate that nature acts for a determinate end. (3) Both animals and plants have their own specific seed to produce their like and not indiscriminate offspring.

      The agent and the matter are not sufficient reasons for the way a thing develops, because nature always proceeds from the same principle to the same end, unless there is some impediment. Just as when a man has a habit of going to a particular place at a particular time it is because of an abiding intention, such as to buy something, so the regularity of nature is for a specific purpose.

      The fact that nature does not deliberate is no argument that it does not act for a purpose, because even art does not deliberate.

The artisan does not deliberate in so far as he possesses the art, but in so far as he fails from the certitude of the art --whence the most certain arts do not deliberate, as the writer does not deliberate as to how he should form the letters. Even those artisans who do deliberate, once they have found the certain principle of the art, do not deliberate in carrying it out. Thus the harpist, if he should deliberate before touching each string, would be considered most inexperienced.

From this it is evident that not to deliberate occurs to some agents, not because they do not act for an end, but because they have determined means by which they act. Whence nature, since it has determinate means through which it acts, for this reason does not deliberate.

For in no other respect does nature seem to differ from art except that nature is an intrinsic principle, and art is an extrinsic principle. For if the art of ship-making were intrinsic in the wood, the ship would be made by nature in the same way that it is now made by art...

Whence it is evident that nature is nothing but a certain kind of art, i.e. the divine art, impressed upon things, by which these things are moved to a determinate end. It is as if the shipbuilder were able to give to timbers that by which they would move themselves to take the form of a ship.


Laplace, Philosophical essay on probabilities:

We ought then to regard the present state of the universe as the effect of its anterior state and as the cause of the one which is to follow. Given for one instant an intelligence which could comprehend all the forces by which nature is animated, and the respective situation of human beings who compose it {an intelligence sufficiently vast to submit these data to analysis} it would embrace in the same formula the movements of the greatest bodies of the universe and those of the lightest atom; for it, nothing would be any more uncertain, and the future and the past would be present to its eyes. The human mind offers, in the perfection which it has been able to give to astronomy, a feeble idea of this intelligence. Its discoveries in mechanics and geometry, added to that of universal gravity, have enabled it to comprehend in the same analytical expressions the past and the future states of the system of the world. Applying the same method to some other objects of its knowledge, it has succeeded in referring to general laws observed phenomena and in foreseeing those which given circumstances ought to produce.

Charles Darwin, The origin of species, ch. 3:
See Thomas Aquinas, Summa Theologiae, I, q. 91, a. 3 (on the body of man)

How is it that the varieties, which I have called incipient species, become ultimately converted into good and distinct species which in most cases obviously differ from each other far more than do the varieties of the same species? How do these groups of species, which constitute what are called distinct genera, and which differ from each other more than do the species of the same genus, arise? All these results follow from the struggle for life. Owing to this struggle, variations, however slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection, in order to mark its relation to man's power of selection. But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient. We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man's feeble efforts, as the works of Nature are to those of Art.

Thomas Aquinas, Summa contra Gentiles, III, ch. 22:

As we said, since any moved thing, in as much as it is moved, tends to the divine likeness so that it may be perfected in itself, and since a thing is perfect in so far as it is actualized, the intention of everything existing in potency must be to tend through motion toward actuality. And so, the more posterior and more perfect an act is, the more fundamentally is the inclination of matter directed toward it. Hence, in regard to the last and most perfect act that matter can attain, the inclination of matter whereby it desires form must be inclined as toward the ultimate end of generation. Now, among the acts pertaining to forms, certain gradations are found. Thus, prime matter is in potency, first of all, to the form of an element. When it is existing under the form of an element it is in potency to the form of a mixed body; that is why the elements are matter for the mixed body. Considered under the form of a mixed body, it is in potency to a vegetative soul, for this sort of soul is the act of a body. In turn, the vegetative soul is in potency to a sensitive soul, and a sensitive one to an intellectual one... After this last type of form no more noble form is found in the order of generable and corruptible things. Therefore, the ultimate end of the whole process of generation is the human soul, and matter tends toward it as toward an ultimate form. So elements exist for the sake of mixed bodies; these latter exist for the sake of living bodies, among which plants exist for animals, and animals for men. Therefore, man is the end of the whole order of generation.

And since a thing is generated and preserved in being by the same reality, there is also an order in the preservation of things, which parallels the foregoing order of generation. Thus we see that mixed bodies are sustained by the appropriate qualities of the elements; plants, in turn, are nourished by mixed bodies; animals get their nourishment from plants; so, those that are more perfect and more powerful from those that are more imperfect and weaker. In fact, man uses all kinds of things for his own advantage: some for food, others for clothing. That is why he was created nude by nature, since he is able to make clothes for himself form other things; just as nature also provided him with no appropriate nourishment except milk, because he can obtain food for himself from a variety of things. Other things he uses for transportation, since we find man the inferior of many animals in quickness of movement and in the strength to do work; other animals being provided, as it were, for his assistance. And, in addition to this, man uses all sense objects for the perfection of intellectual knowledge. Hence it is said of man in the Psalms (8:8) in a statement directed to God: "You have put all things under his feet". And Aristotle says, in Politics I, that man has natural dominion over all animals.

So, if the motion of the heavens is ordered to generation, and if the whole of generation is ordered to man as a last end within this genus, it is clear that the end of celestial motion is ordered to man, as to an ultimate end in the genus of generable and mobile beings.