Wednesday, January 07, 2009

Q67 A3: Whether light is a quality?

Yes. Light is a species of quality 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.

Lux est quaedam qualitas quod lumen habet effectum in natura, quia per radios solis calefiunt corpora. Intentiones autem non causant transmutationes naturales.

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 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.

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 ... quia lumen denominat aerem, fit enim aer luminosus in actu. Color vero non denominat ipsum, non enim dicitur aer coloratus.

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.

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.

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 But light is not the substantial form of air, for if it were, the air would be destroyed when light is withdrawn.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

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.

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.

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.

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.

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.