By Thomas Aquinas
Lectio 2, Q. 5, A. 2 (Translated by Armand Mauer; Toronto, 1953)
Does Natural Philosophy Treat of What Exists in Motion and Matter?
We proceed as follows to the second article:
It seems that natural science does not treat of things that exist in motion and matter, for
1. Matter is the principle of individuation, Now, according to Plato’s doctrine, which is followed by Porphyry, no science treats of individual things but only of universals. Therefore, natural science does not treat of what is in matter.
2. Again, science pertains to the intellect. But the intellect knows by abstracting from matter and from the conditions of matter. Therefore, no science can treat of what is not abstracted from matter.
3. Again, as is clear in the Physics, the First Mover is considered in natural science. But The First Mover is free from all matter. Therefore, natural science does not treat only of what is in matter.
4. Again, every science has to do with what is necessary. But whatever is moved, as such is contingent, as is proved in the Metaphysics. Therefore, no science can treat of what is subject to motion; and so neither can natural science.
5. Again, no universal is subject to motion; for as is said in the beginning of the Metaphysics, it is not man in general who is healed, but this man. But every science concerns that which is universal. Therefore natural science does not treat of what is in motion.
6. Again, some of the things with which natural science deals are not subject to motion; for instance, the soul, as is shown in De Anima, and the earth, as is proved in the De Caelo et Mundo. What is more, all natural forms neither come into being nor perish, and for the same reason they are not subject to motion, except accidentally. This is shown in the Metaphysics. Therefore everything that physics considers is in motion.
7. Again, every creature is mutable for, as Augustine says, true immutability belongs to God alone. So if it is the task of natural science to consider what is in motion, it will be its business to consider all creatures, which clearly appears to be false.
On the contrary, it is the work of natural science to reach conclusions about natural things. Now, natural things are those in which there is a principle of motion; and, as the Metaphysics says, wherever there is motion there must be matter. So natural science treats of what is in motion and matter.
Moreover, these must be some speculative science dealing with what is in matter and motion, for otherwise the teaching of philosophy, which is knowledge of being, would be incomplete. Now no other speculative science treats of these things, for neither mathematics nor metaphysics does so. Therefore, natural science treats of them.
Moreover, the fact is clear from the statements of the Philosopher in the Metaphysics and the Physics.
Reply: It was the difficulty of this problem that drove Plato to posit Ideas. Believing that all sensible things were always in flux, as Cratylus and Heraclitus taught, he thought there can be no science concerning them, as the Philosopher says in the Metaphysics. So he claimed that there were substances separated from the sense world, which might serve as the objects of science and of definitions. He made this mistake because he failed to distinguish what is essential from what is accidental. For it happens that by accident even the wise often fall into error, as is said in the Sophistic Refutations. Now, as is shown in the Metaphysics, we find in a sensible substance both the whole or the composite itself, and also its nature (ratio) or form; and it is the composite that is essentially generated and corrupted and not the nature or form, except accidentally. As the Metaphysics says, “It is not house that is made, but this house.
Now anything can be thought of without all the items that are not essentially related to it. Consequently, forms and natures, though belonging to things existing in motion, are without motion when they are considered in themselves; and so they can be the objects of sciences and of definitions, as the Philosopher says. As he proves, the sciences of sensible reality are not based upon the knowledge of certain substances separated from the sense world.
Natures of this kind, which are the objects of the sciences of real beings, are thought of without motion; and so they must be thought of without those conditions by reason of which motion belongs to mobile things. Now, because every motion is measured by time, and the primary motion is local motion (for without it there is no other motion), a thing must be subject to motion inasmuch as it exists here and now; and it exists under these conditions insofar as it is individuated by matter having determinate dimensions. Consequently, natures of this kind, which make possible sciences of things subject to motion, must be thought of without determinate matter and everything following upon such matter; but not without indeterminate matter, because on its notion depends the notion of form that determines matter to itself. Thus the nature of man, which his definition signifies and which is the object of science, is considered without this flesh and these bones, but not absolutely without flesh and bones. And because individuals include determinate matter in their nature, whereas universals include common matter, as is said in the Metaphysics, the above-mentioned abstraction is not said to be the abstraction of form from matter absolutely, but the abstraction of the universal from the particular.
Natures of this sort, thus abstracted, can be considered in two ways. First, in themselves; and then they are thought of without motion and determinate matter. This happens to them only by reason of the being they have in the intellect. Second, they can be viewed in relation to the things of which they are the natures; and these things exist with matter and motion. Thus they are principles by which we know these things, for everything is known through its form. Consequently, in natural science we know mutable and material things existing outside the soul through natures of this kind; that is to say, natures that are immobile and considered without particular matter.
Replies to opposing arguments:
Reply to 1. Matter is the principle of individuation only insofar as it exists with determinate dimensions, and in this sense natural science indeed abstracts from matter.
Reply to 2. The intelligible form is a thing’s quiddity, for, as the De Anima says, the object of the intellect is the quiddity of a thing. Now, as is said in the Metaphysics, the quiddity of a universal composite, like man or animal, includes within itself common but not particular matter. So the intellect regularly abstracts from determinate matter and its conditions; but in natural science it does not abstract from common matter, although matter itself is considered in natural science only in relation to form. For this reason the natural scientist is more concerned with form than with matter.
Reply to 3. Natural science does not treat of the First Mover as its subject or as part of its subject, but as the end to which natural science leads. Now the end does not belong to the nature of the thing of which it is the end, but it has a relation to it; as the end of a line is not the line but is related to it. So also the First Mover is of a different nature from natural things, but it is related to them because it moves them. So it falls under the consideration of natural science, not in itself, but insofar as it is a mover.
Reply to 4. Science treats of something in two ways: in one way, primarily and principally; and in this sense science is concerned universal natures, which are its very foundation. In another way it treats of something secondarily, as by a sort of reflection; and in this sense it is concerned with the things whose natures they are, inasmuch as, using the lower powers, it relates those natures to the particular things possessing them. For a knower uses a universal nature both as a thing known and as a means of knowing. Thus, through the universal nature of man we can judge of this or that particular man. Now, all universal natures of things are immutable; and so, in this respect, all science is concerned with what is necessary. But some of the things possessing these natures are necessary and immutable, whereas others are contingent and subject to movement, and in this respect sciences are said to be concerned with the contingent and mutable.
Reply to 5. Although a universal is not mutable, it is nevertheless the nature of a mutable thing.
Reply to 6. Although the soul and other natural forms are not themselves subject to motion, they are moved accidentally, and they are, moreover, the perfections of mutable things; and for this reason they come within the domain of natural science. But even though the earth as a whole is not moved (for it happens to be in its natural place, where a thing is at rest in virtue of the same nature through which it is moved to a place), nevertheless, when its parts are outside their proper place, they are moved to a place. Thus the earth falls within the domain of natural science both by reason of the immobility of the whole earth and by reason of the movement of its parts.
Reply to 7. The mutability characteristic of all creatures is not with respect to any natural motion, but with respect to their dependence on God, separation from whom entails destruction of their very being. And that dependence falls under the consideration of metaphysics rather than under that of natural philosophy. Spiritual creatures, moreover, are mutable only with regard to choice; and this sort of motion is not the concern of the natural philosopher but rather of the metaphysician.
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Source: Dominican House of Studies, Super Boethium De Trinitate.
■ Recommended reading:
Boethius and Aquinas, by Ralph McInerny
■ See also: "Boethius," in A History of Western Philosophy Vol. II, by Ralph McInerny. The Jacques Maritain Center.