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The Nature of Light according to Thomas Aquinas


Summa Theologiae, Pars I, q. 67, aa. 1-4

When speaking about light, faithful to his method, Thomas Aquinas’ starting point is terminology: he wants to clarify the use of the word “light” in all its different meanings, hoping to avoid misunderstanding, and this offering us a lesson in a scientific method which seems particularly valid even today (see a.1).  Properly speaking, the original meaning of the word “light” is “something” that makes it possible to see, whereas in another, more analogical, sense, “light” can mean “something” that makes it possible for the other cognitive faculties to obtain the sort of knowledge that is proper to them. The first meaning goes back to the ambit of interaction with the senses, on a physical or material level; the second, on the other hand, analogy brings us into the immaterial ambit of knowledge. Thomas skillfully maintains our attention, for now, on the essential elements of the matter, avoiding getting into a physical theory of light which he deems unnecessary for this initial terminological clarification.

He will address this, however, in the following two parts (see aa.2 e 3), in which we notice that his argumentation is convincing to our modern minds as long as it stays on a purely logical level, but loses our interest when it starts depending on the knowledge obtained from the observable physics of his time. For example, the first argument he calls upon to establish the fact that light is not a body has to do with the property of bodies which cannot occupy the same place as each other at the same time, while he affirms that one light can cross through another in one same place. Today we know that this situation is fundamentally proven by the Pauli exclusion principle, which says that no two matter particles (fermions) can have identical energy states, whereas this does not apply to radiation particles (bosons); in fact, they are two different types of matter that follow different wave functions. The second argument Thomas uses is invalid today, since it is based on the belief of his time that light traveled instantaneously. Regarding his third argumentation based on the notion of “form”, if we were to try to confront it with our present-day knowledge, we would be forced to admit that science has not yet acquired enough of an understanding of “information” (the closest concept we possess that is comparable to Aristotle’s “form”). So we must content ourselves with taking it in its own context.

On the definition of “light”

Whether the word “light” is used in its proper sense in speaking of spiritual things?
Summa theologiae, I, q. 67, a. 1.

I answer that, Any word may be used in two ways – that is to say, either in its original application or in its more extended meaning. This is clearly shown in the word “sight,” originally applied to the act of the sense, and then, as sight is the noblest and most trustworthy of the senses, extended in common speech to all knowledge obtained through the other senses. Thus we say, “Seeing how it tastes,” or “smells,” or “burns.” “Further, sight is applied to knowledge obtained through the intellect, as in those words: “Blessed are the clean of heart, for they shall see God” (Mt 5:8). And thus it is with the word light. In its primary meaning it signifies that which makes manifest to the sense of sight; afterwards it was extended to that which makes manifest to cognition of any kind. If, then, the word is taken in its strict and primary meaning, it is to be understood metaphorically when applied to spiritual things, as Ambrose says (De Fide II). But if taken in its common and extended use, as applied to manifestation of every kind, it may properly be applied to spiritual things.

The answer to the objections will sufficiently appear from what has been said.


Whether light is a body or a quality

Whether light is a body?
Summa theologiae
, I, q. 67, a. 2, co:

I answer that, Light cannot be a body, for three evident reasons. First, on the part of place. For the place of any one body is different from that of any other, nor is it possible, naturally speaking, for any two bodies of whatever nature, to exist simultaneously in the same place; since contiguity requires distinction of place.

The second reason is from movement. For if light were a body, its diffusion would be the local movement of a body. Now no local movement of a body can be instantaneous, as everything that moves from one place to another must pass through the intervening space before reaching the end: whereas the diffusion of light is instantaneous. Nor can it be argued that the time required is too short to be perceived; for though this may be the case in short distances, it cannot be so in distances so great as that which separates the East from the West. Yet as soon as the sun is at the horizon, the whole hemisphere is illuminated from end to end. It must also be borne in mind on the part of movement that whereas all bodies have their natural determinate movement, that of light is indifferent as regards direction, working equally in a circle as in a straight line. Hence it appears that the diffusion of light is not the local movement of a body.

The third reason is from generation and corruption. For if light were a body, it would follow that whenever the air is darkened by the absence of the luminary, the body of light would be corrupted, and its matter would receive a new form. But unless we are to say that darkness is a body, this does not appear to be the case. Neither does it appear from what matter a body can be daily generated large enough to fill the intervening hemisphere. Also it would be absurd to say that a body of so great a bulk is corrupted by the mere absence of the luminary. And should anyone reply that it is not corrupted, but approaches and moves around with the sun, we may ask why it is that when a lighted candle is obscured by the intervening object the whole room is darkened? It is not that the light is condensed round the candle when this is done, since it burns no more brightly then than it burned before.

Since, therefore, these things are repugnant, not only to reason, but to common sense, we must conclude that light cannot be a body.


Whether light is a quality?
Summa theologiae, I, q. 67, a. 3.

I answer that, Some writers have said that the light in the air has not a natural being such as the color on a wall has, but only an intentional being, as a similitude of color in the air. But this cannot be the case for two reasons. First, because light gives a name to the air, since by it the air becomes actually luminous. But color does not do this, for we do not speak of the air as colored. Secondly, because light produces natural effects, for by the rays of the sun bodies are warmed, and natural changes cannot be brought about by mere intentions. Others have said that light is the sun's substantial form, but this also seems impossible for two reasons. First, because substantial forms are not of themselves objects of the senses; for the object of the intellect is what a thing is, as is said De Anima III, text. 26: whereas light is visible of itself. In the second place, because it is impossible that what is the substantial form of one thing should be the accidental form of another; since substantial forms of their very nature constitute species: wherefore the substantial form always and everywhere accompanies the species. But light is not the substantial form of air, for if it were, the air would be destroyed when light is withdrawn. Hence it cannot be the substantial form of the sun.

We must say, then, that as heat is an active quality consequent on the substantial form of fire, so light is an active quality consequent on the substantial form of the sun, or of another body that is of itself luminous, if there is any such body. A proof of this is that the rays of different stars produce different effects according to the diverse natures of bodies.


Why God created light the first day

The following text of the Summa Theologica addresses the Biblical cosmogony in which light is the first creature of what we now call the physical world to be created.  Nothing at all is said here about the creation of the angels or of anything else outside the material realm. Thomas examines, as he does in other places, the opinions of the great authors who have gone before him and, given that he is analyzing a Biblical text, he refers naturally to the Fathers and Doctors of the Church: Augustine, Basil, and John Chrysostom. He then proceeds to add to their opinions, his own more philosophical argument, based on Aristotelian hylomorphism (a philosophical view which conceives being as a compound of matter and form) adapting it somewhat to the exegesis of this Scripture passage. He almost seems to say that the uninformed matter just created was a sort of prime matter, perhaps already “marked by quantity”  (signata quantitate), from which the Creator brought forth light as its first form. If we were to try something similar today with our knowledge of physics, concordism aside, we would be led to think of the creation of the quantic vacuum, from which, through a flow induced by some unknown cause, the first real particles emerged, massless and thus swift as light, until the Higgs field effect stopped them. Thomas seems not a bit less courageous: if, as we see, he projects an interpretation of the text that clearly bears the mark of the exegetical limitations of his time, he shows a sincere desire to look at the real world, which, he knows, was created by God.


Whether the production of light is fittingly assigned to the first day?
Summa theologiae
, I, q. 67, a. 4.

I answer that, There are two opinions as to the production of light. Augustine seems to say (De Civtate Dei XI, 9,33) that Moses could not have fittingly passed over the production of the spiritual creature, and therefore when we read, “In the beginning God created heaven and earth,” a spiritual nature as yet formless is to be understood by the word “heaven,” and formless matter of the corporeal creature by the word “earth.” And spiritual nature was formed first, as being of higher dignity than corporeal. The forming, therefore, of this spiritual nature is signified by the production of light, that is to say, of spiritual light. For a spiritual nature receives its form by the enlightenment whereby it is led to adhere to the Word of God.

Other writers think that the production of spiritual creatures was purposely omitted by Moses, and give various reasons. Basil [*Homilia I in Hexaemeron] says that Moses begins his narrative from the beginning of time which belongs to sensible things; but that the spiritual or angelic creation is passed over, as created beforehand.

Chrysostom [Homilia II in Genesim] gives as a reason for the omission that Moses was addressing an ignorant people, to whom material things alone appealed, and whom he was endeavoring to withdraw from the service of idols. It would have been to them a pretext for idolatry if he had spoken to them of natures spiritual in substance and nobler than all corporeal creatures; for they would have paid them Divine worship, since they were prone to worship as gods even the sun, moon, and stars, which was forbidden them (Dt 4).

But mention is made of several kinds of formlessness, in regard to the corporeal creature. One is where we read that “the earth was void and empty,” and another where it is said that “darkness was upon the face of the deep.” Now it seems to be required, for two reasons, that the formlessness of darkness should be removed first of all by the production of light. In the first place because light is a quality of the first body, as was stated (a. 3), and thus by means of light it was fitting that the world should first receive its form. The second reason is because light is a common quality. For light is common to terrestrial and celestial bodies. But as in knowledge we proceed from general principles, so do we in work of every kind. For the living thing is generated before the animal, and the animal before the man, as is shown in De Gener. Anim. II, 3. It was fitting, then, as an evidence of the Divine wisdom, that among the works of distinction the production of light should take first place, since light is a form of the primary body, and because it is more common quality.

Basil [Homilia II in Hexaemeron], indeed, adds a third reason: that all other things are made manifest by light. And there is yet a fourth, already touched upon in the objections; that day cannot be unless light exists, which was made therefore on the first day.


Objections and replies

1: It would seem that the production of light is not fittingly assigned to the first day. For light, as stated above (a. 3), is a quality. But qualities are accidents, and as such should have, not the first, but a subordinate place. The production of light, then, ought not to be assigned to the first day.

Ad 1um According to the opinion of those who hold that the formlessness of matter preceded its form in duration, matter must be held to have been created at the beginning with substantial forms, afterwards receiving those that are accidental, among which light holds the first place.

2: Further, it is light that distinguishes night from day, and this is effected by the sun, which is recorded as having been made on the fourth day. Therefore the production of light could not have been on the first day.

Ad 2um  In the opinion of some the light here spoken of was a kind of luminous nebula, and that on the making of the sun this returned to the matter of which it had been formed. But this cannot well be maintained, as in the beginning of Genesis Holy Scripture records the institution of that order of nature which henceforth is to endure. We cannot, then, say that what was made at that time afterwards ceased to exist.

Others, therefore, held that this luminous nebula continues in existence, but so closely attached to the sun as to be indistinguishable. But this is as much as to say that it is superfluous, whereas none of God's works have been made in vain. On this account it is held by some that the sun's body was made out of this nebula. This, too, is impossible to those at least who believe that the sun is different in its nature from the four elements, and naturally incorruptible. For in that case its matter cannot take on another form.

I answer, then, with Dionysius (Divinis Nominibus IV), that the light was the sun's light, formless as yet, being already the solar substance, and possessing illuminative power in a general way, to which was afterwards added the special and determinative power required to produce determinate effects. Thus, then, in the production of this light a triple distinction was made between light and darkness. First, as to the cause, forasmuch as in the substance of the sun we have the cause of light, and in the opaque nature of the earth the cause of darkness.

Secondly, as to place, for in one hemisphere there was light, in the other darkness. Thirdly, as to time; because there was light for one and darkness for another in the same hemisphere; and this is signified by the words, “He called the light day, and the darkness night.”

3: Further, night and day are brought about by the circular movement of a luminous body. But movement of this kind is an attribute of the firmament, and we read that the firmament was made on the second day. Therefore the production of light, dividing night from day, ought not to be assigned to the first day.

Ad 3um Basil says (Homilia II in Hexaemeron) that day and night were then caused by expansion and contraction of light, rather than by movement. But Augustine objects to this (Gen. ad lit. I), that there was no reason for this vicissitude of expansion and contraction since there were neither men nor animals on the earth at that time, for whose service this was required. Nor does the nature of a luminous body seem to admit of the withdrawal of light, so long as the body is actually present; though this might be effected by a miracle. As to this, however, Augustine remarks (Gen. ad lit. i) that in the first founding of the order of nature we must not look for miracles, but for what is in accordance with nature. We hold, then, that the movement of the heavens is twofold. Of these movements, one is common to the entire heaven, and is the cause of day and night. This, as it seems, had its beginning on the first day. The other varies in proportion as it affects various bodies, and by its variations is the cause of the succession of days, months, and years. Thus it is, that in the account of the first day the distinction between day and night alone is mentioned; this distinction being brought about by the common movement of the heavens. The further distinction into successive days, seasons, and years recorded as begun on the fourth day, in the words, "let them be for seasons, and for days, and years" is due to proper movements.

4: Further, if it be said that spiritual light is here spoken of, it may be replied that the light made on the first day dispels the darkness. But in the beginning spiritual darkness was not, for even the demons were in the beginning good, as has been shown (q. 63, a. 5). Therefore the production of light ought not to be assigned to the first day.

Ad 4um As Augustine teaches (Confessiones XII; Genesi ad litteram 1,15), formlessness did not precede forms in duration; and so we must understand the production of light to signify the formation of spiritual creatures, not, indeed, with the perfection of glory, in which they were not created, but with the perfection of grace, which they possessed from their creation as said above (q. 62, a. 3). Thus the division of light from darkness will denote the distinction of the spiritual creature from other created things as yet without form. But if all created things received their form at the same time, the darkness must be held to mean the spiritual darkness of the wicked, not as existing from the beginning but such as God foresaw would exist. 


Source for the digital English text: Christian Classic Ethereal Library