Plotino - Tratado 29,6 (IV, 5, 6) — Há luz sem ar?

6. Si la luz pudiese producirse sin intervención del aire, el sol iluminaría asimismo la superficie de los cuerpos, aun en el supuesto de que el aire, ahora iluminado por accidente, fuese reemplazado por el vacío. Pero si las demás cosas son iluminadas por serlo también el aire, la existencia de la luz debe atribuirse al aire, no siendo entonces otra cosa que una afección de éste. No existiría, pues, en definitiva, de no darse asimismo la existencia del aire afectado.

Digamos, ante todo, que la luz no es en su origen   una afección del aire, ni, por supuesto, del aire como tal aire. Es propia, por el contrario, de todo cuerpo ígneo o brillante; tanto es así, que las piedras brillantes tienen el color de la luz. Pero, ¿se daría la luz, en el paso de estos cuerpos que poseen tal color a otro cuerpo, si no se diese a la vez el aire? Si la luz es sólo una cualidad, y una cualidad de un determinado ser, puesto que toda cualidad ha de existir necesariamente en un sujeto, hemos de inquirir ciertamente en qué cuerpo se encuentra la luz. Pero si es, en cambio, una actividad surgida de algún cuerpo, ¿por qué no podría existir en la vecindad de éste, en un espacio intermedio y vacío que permitiese su propagación? Porque es claro que si la luz se extiende en línea recta, podrá continuar su marcha sin cabalgar sobre ningún ser.

Si lo propio de la luz fuese caer, caería necesariamente. Y entonces ni el aire, ni, en general, ningún cuerpo iluminado, la arrancarían del cuerpo que la produce y la obligarían a avanzar. Pero no es un accidente, que deba darse en absoluto en otra cosa, ni tampoco una afección, que exigiría un cuerpo afectado. De modo que, o deberá permanecer en el sujeto cuando ya se ha ido la fuente productora de la luz, o ella misma ha de irse con su fuente. Pero, así, vendría también consigo misma. ¿Cómo? Sería suficiente que contase con un espacio. En otro caso, el cuerpo del sol perdería su propia actividad. Y esta actividad no es otra que la luz.

El que una actividad provenga de un sujeto no hace suponer que termine en otro sujeto. Si éste se presenta, experimentará como tal sujeto una determinada afección. Pero así como la vida es una actividad del alma, que confluye en un cuerpo, si éste se presenta, pero que sigue existiendo aunque el cuerpo no esté a su alcance ¿qué impide que ocurra lo mismo con la luz, si ella es también la actividad de un cuerpo luminoso? Porque no es la oscuridad del aire la que engendra la luz, sino que es su misma mezcla con la tierra la que hace a la luz oscura y verdaderamente impura. Decir que el aire la produce es tanto como afirmar que una cosa es dulce por su mezcla con otra amarga. Si se dice, pues, que la luz es una modificación del aire, habrá que añadir además que esta modificación afecta también al aire, cuya oscuridad sufre una transformación y deja de ser ya oscuridad. El aire, cuando está iluminado, permanece tal cual es sin ser afectado por nada. La afección pertenece tan sólo al objeto en el que se da, bien entendido que el color no es una afección del aire, sino que existe por sí mismo; así, decimos del color que está presente en él. Y con esto se ha tratado bastante de la cuestión.

VI. Il nous reste à examiner s’il y aurait de la lumière sans air, dans le cas où le soleil illuminerait la surface des corps et que le vide existerait dans l’intervalle qui est actuellement éclairé par accident en vertu de la place qu’il occupe [entre le soleil et les corps]. — Certes, si les autres choses étaient affectées parce que l’air serait affecté lui-même) si la lumière avait pour substance l’air dont elle serait une affection, cette affection ne saurait exister sans le sujet qui l’éprouverait. Mais [selon nous], la lumière n’est pas essentiellement propre à l’air en tant qu’air : car tous les corps ignés et brillants, entre autres les pierres précieuses, ont une couleur lumineuse. — Ce qui passe d’un corps brillant dans un autre corps existerait-il sans cet autre corps? — Si la lumière n’est qu’une simple qualité d’une chose, comme toute qualité suppose un sujet, il faut chercher la lumière dans le corps dans lequel elle réside. Si c’est 420 au contraire un acte produit par une autre chose, pourquoi, s’il n’y a pas de corps auprès de l’objet lumineux, s’il n’y a que le vide, la lumière n’existerait-elle pas et ne s’étendrait-elle pas aussi au-dessus [comme au-dessous, en rayonnant en tout sens]? Puisqu’elle s’étend, pourquoi ne se répandrait-elle pas sans être arrêtée? Si sa nature est de tomber, elle s’abaissera d’elle-même : car ni l’air ni aucun corps éclairé ne la fera sortir du corps éclairant et ne la forcera de s’avancer, puisqu’elle n’est pas un accident qui suppose un sujet, ni une affection qui suppose un objet affecté. Sans cela, la lumière demeurerait [dans le corps éclairé] quand l’objet dont elle émane viendrait à s’éloigner; mais elle s’éloigne avec lui ; elle s’étend donc. Où s’étend- elle? Il suffit que le lieu existe [pour qu’elle s’étende] ; sinon, le corps du soleil perdrait son acte, c’est-à-dire la lumière qu’il répand. S’il en est ainsi, la lumière n’est pas la qualité d’un sujet; c’est l’acte qui émane d’un sujet, mais ne passe pas dans un autre sujet (18) ; seulement, si un autre sujet est présent, il éprouvera une affection. Comme la vie, qui constitue un acte de l’âme, affecte le corps s’il est présent, et n’en constitue pas moins un acte si le corps est absent, de même, la lumière constitue un acte soumis aux mêmes conditions. Ce n’est pas le ténébreux de l’air qui engendre la lumière, ni le ténébreux mêlé à la terre qui produit une lumière impure ; sinon, on pourrait produire le doux en mêlant une chose à l’amer. Si donc on dit que la lumière est une modification (τροπή) de l’air, il faut ajouter que l’air doit être modifié lui-même par cette modification, et que le ténébreux de l’air n’est plus ténébreux après avoir subi ce changement. Quant à l’air, il reste ce qu’il était, comme s’il n’avait pas été affecté. L’affection n’appartient qu’à ce qui est affecté. La couleur n’appartient 421 donc pas à l’air, mais subsiste en elle-même; l’air est seulement présent. En voici assez sur ce sujet.

THE RELATION OF THE AIR TO THE LIGHT.

6. Could light exist without air, if the sun illuminated the surface of bodies, and if there were a void in the interval which is accidentally illuminated by virtue of its location (between the sun and the bodies) ? It is certain that if the other things were affected because the air itself was affected, and if light were nothing more than an affection of the air, that is, its substance; then indeed this affection could not exist without the experiencing subject (the air). But (in our view) light is not essentially characteristic of air as such; for all fiery and brilliant bodies, among which are precious stones, possess a luminous color. Could that which passes from a brilliant body into some other body exist without that other body? If light be but a simple quality of an object, and as every quality implies a subject on which it depends, light will have to be sought in the body in which it resides. If, on the contray, light be only an actualization produced by some other thing, and if there be no body contiguous to the luminous object, and it be entirely surrounded by a void, why could light not exist, and radiate upwards (as well as downwards, and in every direction) ? Since light radiates, why should it not radiate without hindrance? If its nature be to fall, it will spontaneously descend; for neither the air nor any illuminated body will make it issue from the illuminating body, nor can force it to advance, since it is neither an accident that implies a subject, nor an affection that implies an affected object. Otherwise, the light would remain (in the illuminated body) when the object from which it emanates should happen to withdraw; but since the light withdraws with it, it radiates. In what direction does light radiate? (Its radiation) demands no more than the existence of sufficient space; otherwise the body of the sun would lose its actualization; that is, the light it radiates. In this case light would not be the quality of a subject, but the actualization that emanates from a subject, but which does not pass into any other subject (as a kind of undulation); but if another subject be present, it will suffer an affection. As life, which constitutes an actualization of the soul, affects the body if it be present, and does not any the less constitute an actualization if the body be absent, likewise light constitutes an actualization subject to the same conditions. It is not the obscurity of the air that begets light, nor obscurity mingled with the earth which produces an impure light; otherwise one might produce something sweet by mingling some thing with what is bitter. The statement that light is a modification of the air, is incomplete without the addition that the air must itself be modified by this modification, and that the obscurity of the air is no longer obscure after having undergone that change. The air itself, however, remains what it was, just as if it had not been affected. The affection belongs only to that which has been affected. Color therefore does not belong to the air, but subsists in itself; the air’s only function is its presence. But enough of this.

6. We return, then, to the question whether there could be light if there were no air, the sun illuminating corporeal surfaces across an intermediate void which, as things are, takes the light accidentally by the mere fact of being in the path. Supposing air to be the cause of the rest of things being thus affected, the substantial existence of light is due to the air; light becomes a modification of the air, and of course if the thing to be modified did not exist neither could be modification.

The fact is that primarily light is no appanage of air, and does not depend upon the existence of air: it belongs to every fiery and shining body, it constitutes even the gleaming surface of certain stones.

Now if, thus, it enters into other substances from something gleaming, could it exist in the absence of its container?

There is a distinction to be made: if it is a quality, some quality of some substance, then light, equally with other qualities, will need a body in which to lodge: if, on the contrary, it is an activity rising from something else, we can surely conceive it existing, though there be no neighbouring body but, if that is possible, a blank void which it will overleap and so appear on the further side: it is powerful, and may very well pass over unhelped. If it were of a nature to fall, nothing would keep it up, certainly not the air or anything that takes its light; there is no reason why they should draw the light from its source and speed it onwards.

Light is not an accidental to something else, requiring therefore to be lodged in a base; nor is it a modification, demanding a base in which the modification occurs: if this were so, it would vanish when the object or substance disappeared; but it does not; it strikes onward; so, too [requiring neither air nor object] it would always have its movement.

But movement, where?

Is space, pure and simple, all that is necessary?

With unchecked motion of the light outward, the material sun will be losing its energy, for the light is its expression.

Perhaps; and [from this untenable consequence] we may gather that the light never was an appanage of anything, but is the expressive Act proceeding from a base [the sun] but not seeking to enter into a base, though having some operation upon any base that may be present.

Life is also an Act, the Act of the soul, and it remains so when anything - the human body, for instance - comes in its path to be affected by it; and it is equally an Act though there be nothing for it to modify: surely this may be true of light, one of the Acts of whatever luminary source there be [i.e., light, affecting things, may be quite independent of them and require no medium, air or other]. Certainly light is not brought into being by the dark thing, air, which on the contrary tends to gloom it over with some touch of earth so that it is no longer the brilliant reality: as reasonable to talk of some substance being sweet because it is mixed with something bitter.

If we are told that light is a mode of the air, we answer that this would necessarily imply that the air itself is changed to produce the new mode; in other words, its characteristic darkness must change into non-darkness; but we know that the air maintains its character, in no wise affected: the modification of a thing is an experience within that thing itself: light therefore is not a modification of the air, but a self-existent in whose path the air happens to be present.

On this point we need dwell no longer; but there remains still a question.