Chapter Thirteen In place of this chapter I have chosen to insert the following two letters, one a response and the other a refutation, concerning the question whether light is an accident or a substance. I include them for the question’s own sake; because the second letter has been mentioned in discussing the preceding question; and because it contains several other matters bearing on my undertaking, especially what I intend by “substance” in this little treatise.

First Letter Sir, The day before yesterday you sent a syllogistic argument to be put to our physician, R., drawn from the question in natural philosophy whether light is a substance or an accident. It takes this form: Whatever is created through the Word of God is a substance. But light is created through the Word of God. Therefore light is a substance.

I replied at once that the argument could not stand. My reason was that the major premise, which you considered self-evidently and universally true, seemed to me only particular. Yet in the first figure—to answer initially as to form—the major premise must not be particular. We must consider that God also created accidents involving no imperfection or corruption: attributes, properties, qualities, and adjuncts of things, such as colour, heat, moisture, and cold. These are accidents that attach to or inhere in substances in their proper ways. Otherwise we would be trying to overthrow a philosophical principle never denied by any philosopher: whatever exists is either a substance or an accident, with no intermediate between them.

To make the matter clearer in that note, briefly and by one example, I adduced a kind of accident: the image of God created together with man. It does not enter into man’s essence, neither his form nor his matter; it therefore does not constitute a third intrinsic cause of substance, for no such cause exists in Nature. This example could show that, if not everything created by God’s Word is a substance, the major premise would be only particular, and no certain conclusion could follow from it in the first figure.

If this error were encouraged and accepted, I wrote, one could readily conclude that what took the place of the divine image after the Fall—namely sin—was also a substance. This is the opinion of the Flacians: Whatever is in man is a substance. Sin is in man. Therefore sin is a substance.

One error gives rise to another, that of certain Calvinists: if not directly, then by an assured consequence, God would be the author or creator of sin and subsequently of damnation, according to this argument: God is the author of all substances. Sin is a substance. Therefore God is the author of sin.

Although I know that you neither hold nor assent to these errors, they should have served partly as a reduction to absurdity, and partly to overthrow the universality of the major term: not everything, without exception, created by God or by his Word is a substance. There has therefore been no shift from the question at issue—whether light is a substance or an accident and quality—to a different controversy concerning the image of God.

Further, I oppose to your argument the following: Whatever inheres in a body is an accident. Light inheres in a body. Therefore light is an accident. Or in this form: Every substance subsists of itself. Light does not subsist of itself. Therefore light is not a substance.

The major premise of the first syllogism is proved because an accident does not subsist outside its subject. The minor premise of the second is proved because this is the defining character of accidents: they do not subsist of themselves. You will readily grant both points, since they rest upon accepted and unassailable terms and principles of philosophy.

The minor premise of the first is proved because a body is and remains a body in itself without light attached to it, even in the deepest darkness. Otherwise it would follow that light constituted the body, either materially, as that from which it exists, or formally, as that through which it exists. As for the minor premise of the second: light exists in fire, in the sun and stars, or in some other body, and not in and through itself.

Further, if light is a substance, it is either corporeal or incorporeal, for there is no third possibility. It is not corporeal, since otherwise there would be a body within a body, which is absurd. Nor is it incorporeal, since it is seen; in this frail mortal condition nothing incorporeal is perceptible to our eyes and sight. Therefore it is not a substance.

To make the matter clearer, it must be understood that I mean light in its proper and natural sense, as perceived through the eyes and organs of sight by the faculty of seeing. I maintain that it is an accident, quality, and most subtle affection of a luminous body, by which the world and its various bodies become perceptible to man. It varies according to its subjects. First, it may be extraordinary, as was the primordial light during the first three days, before the heavenly bodies were created.