Thomas Aquinas, Summa Theologiae

Objection 1: It would seem that light is not a quality. For every quality remains in its subject, though the active cause of the quality be removed, as heat remains in water removed from the fire. But light does not remain in the air when the source of light is withdrawn. Therefore light is not a quality.

Objection 2: Further, every sensible quality has its opposite, as cold is opposed to heat, blackness to whiteness. But this is not the case with light since darkness is merely a privation of light. Light therefore is not a sensible quality.

Objection 3: Further, a cause is more potent than its effect. But the light of the heavenly bodies is a cause of substantial forms of earthly bodies, and also gives to colors their immaterial being, by making them actually visible. Light, then, is not a sensible quality, but rather a substantial or spiritual form.

On the contrary, Damascene (De Fide Orth. i) says that light is a species of quality.

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 But light is not the substantial form of air, for if it were, the air would be destroyed when light is withdrawn.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.

Reply to Objection 1: Since quality is consequent upon substantial form, the mode in which the subject receives a quality differs as the mode differs in which a subject receives a substantial form. For when matter receives its form perfectly, the qualities consequent upon the form are firm and enduring; as when, for instance, water is converted into fire. When, however, substantial form is received imperfectly, so as to be, as it were, in process of being received, rather than fully impressed, the consequent quality lasts for a time but is not permanent; as may be seen when water which has been heated returns in time to its natural state. But light is not produced by the transmutation of matter, as though matter were in receipt of a substantial form, and light were a certain inception of substantial form. For this reason light disappears on the disappearance of its active cause.

Reply to Objection 2: It is accidental to light not to have a contrary, forasmuch as it is the natural quality of the first corporeal cause of change, which is itself removed from contrariety.

Reply to Objection 3: As heat acts towards perfecting the form of fire, as an instrumental cause, by virtue of the substantial form, so does light act instrumentally, by virtue of the heavenly bodies, towards producing substantial forms; and towards rendering colors actually visible, inasmuch as it is a quality of the first sensible body.

Articulus 3

Argumentum 1

Ad tertium sic proceditur. Videtur quod lux non sit qualitas.

Omnis enim qualitas permanet in subiecto etiam postquam agens discesserit; sicut calor in aqua postquam removetur ab igne.

Sed lumen non remanet in aere recedente luminari.

Ergo lumen non est qualitas.

Argumentum 2

Praeterea, omnis qualitas sensibilis habet contrarium; sicut calido contrariatur frigidum, et albo nigrum. Sed lumini nihil est contrarium, tenebra enim est privatio luminis.

Ergo lumen non est qualitas sensibilis.

Argumentum 3

Praeterea, causa est potior effectu. Sed lux caelestium corporum causat formas substantiales in istis inferioribus. Dat etiam esse spirituale coloribus, quia facit eos visibiles actu. Ergo lux non est aliqua qualitas sensibilis, sed magis substantialis forma, aut spiritualis.

Sed Contra

Sed contra est quod Damascenus dicit, in libro I, quod lux est quaedam qualitas.

Corpus

Respondeo dicendum quod quidam dixerunt quod lumen in aere non habet esse naturale, sicut color in pariete; sed esse intentionale, sicut similitudo coloris in aere.

Sed hoc non potest esse, propter duo. Primo quidem, quia lumen denominat aerem, fit enim aer luminosus in actu. Color vero non denominat ipsum, non enim dicitur aer coloratus. Secundo, quia lumen habet effectum in natura, quia per radios solis calefiunt corpora. Intentiones autem non causant transmutationes naturales.

Alii vero dixerunt quod lux est forma substantialis solis.

Sed hoc etiam apparet impossibile, propter duo. Primo quidem, quia nulla forma substantialis est per se sensibilis, quia quod quid est est obiectum intellectus, ut dicitur in III de anima.

Lux autem est secundum se visibilis. Secundo, quia impossibile est ut id quod est forma substantialis in uno, sit forma accidentalis in alio, quia formae substantiali per se convenit constituere in specie; unde semper et in omnibus adest ei. Lux autem non est forma substantialis aeris, alioquin, ea recedente, corrumperetur.

Unde non potest esse forma substantialis solis. Dicendum est ergo quod, sicut calor est qualitas activa consequens formam substantialem ignis, ita lux est qualitas activa consequens formam substantialem solis, vel cuiuscumque alterius corporis a se lucentis, si aliquod aliud tale est. Cuius signum est, quod radii diversarum stellarum habent diversos effectus, secundum diversas naturas corporum.

Ad 1

Ad primum ergo dicendum quod, cum qualitas consequatur formam substantialem, diversimode se habet subiectum ad receptionem qualitatis, sicut se habet ad receptionem formae. Cum enim materia perfecte recipit formam, firmiter stabilitur etiam qualitas consequens formam; sicut si aqua convertatur in ignem. Cum vero forma substantialis recipitur imperfecte, secundum inchoationem quandam, qualitas consequens manet quidem aliquandiu, sed non semper; sicut patet in aqua calefacta, quae redit ad suam naturam.

Sed illuminatio non fit per aliquam transmutationem materiae ad susceptionem formae substantialis, ut fiat quasi inchoatio aliqua formae. Et ideo lumen non remanet nisi ad praesentiam agentis.

Ad 2

Ad secundum dicendum quod accidit luci quod non habeat contrarium, inquantum est qualitas naturalis primi corporis alterantis, quod est a contrarietate elongatum.

Ad 3

Ad tertium dicendum quod, sicut calor agit ad formam ignis quasi instrumentaliter in virtute formae substantialis, ita lumen agit quasi instrumentaliter in virtute corporum caelestium ad producendas formas substantiales, et ad hoc quod faciat colores visibiles actu, inquantum est qualitas primi corporis sensibilis.