Thomas Aquinas

translated by John P. Rowan
Chicago, 1961

html-edited by Joseph Kenny, O.P.



Introduction (1-68)


The work itself (69-2663)

What previous philosophers said about causes (69-272)

Determination of the truth (273-2663)

BOOK 2—α

With regard to universal truth (274-337)

With regard to the truth about what belongs to this science (338-2663)


Dialectical exposition of the problems (338-528)

Demonstrative section 529-2663


List of things this science considers (529-748)

Determination of these things (749-2663)


The meanings of the terms used in this science (749-1143)

Names signifying causes (749-841)

Names signifying the subject of this science (842-1032)

Names signifying attributes or aspects of the subject (1033-1143)

Determination of the realities this science considers (1144-2663)


The method of considering "being" in this science (1144-1244)

The truth about "being" (1245-2663)


The truth about "being as being" (1245-2145)

"Being" (1245-1919)

As it is divided by the ten categories (1245-1767)

The need to focus on substance (1245-1269)

The truth about substance (1270-1767)

The method and order of discussion (1270-1305)

Sensible substances (1306-1767)

General and logical considerations (1306-1680)

Considering the principles of sensible substances (1681-1767)


As it is divided by potency and act (1768-1919)


"One" and its concomitants (1920-2145)


"One" in itself (1920-1982)

In comparison with multitude (1983-2145)

The first principles of being: separated substances (2146-2663)


Preliminary considerations (2146-2415)

Separated substances (2416-2663)



When several things are ordained to one thing, one of them must rule or govern and the rest be ruled or governed, as the Philosopher, teaches in the Politics. This is evident in the union of soul and body, for the soul naturally commands and the body obeys. The same thing is true of the soul’s powers, for the concupiscible and irascible appetites are ruled in a natural order by reason. Now all the sciences and arts are ordained to one thing, namely, to man’s perfection, which is happiness. Hence one of these sciences and arts must be the mistress of all the others, and this rightly lays claim to the name wisdom; for it is the office of the wise man to direct others.

We can discover which science this is and the sort of things with which it deals by carefully examining the qualities of a good ruler; for just as men of superior intelligence are naturally the rulers and masters of others, whereas those of great physical strength and little intelligence are naturally slaves, as the Philosopher says in the aforementioned book in a similar way that science which is intellectual in the highest degree should be naturally the ruler of the others. This science is the one which treats of the most intelligible objects.

Now the phrase “most intelligible objects” can be understood in three ways. First, from the viewpoint of the order of knowing; for those things from which the intellect derives certitude seem to be more intelligible. Therefore, since the certitude of science is acquired by the intellect knowing causes, a knowledge of causes seems to be intellectual in the highest degree. Hence that science which considers first causes also seems to be the ruler of the others in the highest degree.

Second, this phrase can be understood by comparing the intellect with the senses; for while sensory perception is a knowledge of particulars, the intellect seems to differ from sense by reason of the fact that it comprehends universals. Hence that science is pre-eminently intellectual which deals with the most universal principles. These principles are being and those things which naturally accompany being, such as unity and plurality, potency and act. Now such principles should not remain entirely undetermined, since without them complete knowledge of the principles which are proper to any genus or species cannot be had. Nor again should they be dealt with in any one particular science, for, since a knowledge of each class of beings stands in need if such principles, they would with equal reason be investigated in every particular science. It follows, then, that such principles should be treated by one common science, which, since it is intellectual in the highest degree, is the mistress of the others.

Third, this phrase can be understood from the viewpoint of the intellect’s own knowledge. For since each thing has intellective power by virtue of being free from matter, those things must be intelligible in the highest degree which are altogether separate, from matter. For the intellect and the intelligible object must be proportionate to each other and must belong to the same genus, since the intellect and the intelligible object are one in act. Now those things are separate from matter in the highest degree which abstract not only from signate matter (as the natural forms taken universally of which the philosophy of nature treats) but from sensible matter altogether; and these are separate from matter not only in their intelligible constitution (ratio), as the objects of mathematics, but also in being (esse), as God and the intelligences. Therefore the science which considers such things seems to be the most intellectual and the ruler or mistress of the others.

Now this threefold consideration should be assigned to one and the same science and not to different sciences, because the aforementioned separate substances are the universal and first causes of being. Moreover, it pertains to one and the same science to consider both the proper causes of some genus and the genus itself; for example, the philosophy of nature considers the principles of a natural body. Therefore, it must be the office of one and the same science to consider the separate substances and being in general (ens commune), which is the genus of which the aforementioned substances are the common and universal causes.

From this it is evident that, although this science (metaphysics or first philosophy) studies the three things mentioned above, it does not investigate any one of them as its subject, but only being in general. For the subject of a science is the genus whose causes and properties we seek, and not the causes themselves of the particular genus studied; for a knowledge of the causes of some genus is the goal to which the investigation of a science attains. Now although the subject of this science is being in general, the whole of it is predicated of those things which are separate from matter both in their intelligible constitution and in being. For it is not only those things which can never exist in matter that are said to be separate from matter in their intelligible constitution and being, such as God and the intellectual substances, but also those which can exist without matter, as being in general. This could not be the case, however, if their being depended on matter.

Therefore in accordance with the three things mentioned above from which this science derives its perfection, three names arise. It is called divine science or theology inasmuch as it considers the aforementioned substances. It is called metaphysics inasmuch as it considers being and the attributes which naturally accompany being (for things which transcend the physical order are discovered by the process of analysis, as the more common are discovered after the less common). And it is called first philosophy inasmuch as it considers the first causes of things. Therefore it is evident what the subject of this science is, and how it is related to the other sciences, and by what names it is designated.