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It is permissible for us to assert that a transparent body receives something from sight and transmits it to the visible object and that perception occurs through the unbroken succession of that thing to the visible object, between the eye and the visible object. And this is the view of those who suppose that rays issue from the eye. Therefore, [for the sake of argument,] let it be supposed that this is true, that rays issue from the eye and pass through the transparent medium to the object of sight and that perception occurs by means of· those rays. And since, [according to the view now under consideration,] perception occurs in this way, I inquire whether those rays return something to the eye. If perception occurs [only] through [such] rays, and they do not return anything to the eye, then the eye does not perceive. But the eye does perceive the object of sight, and [we have supposed that] it perceives only by the mediation of rays. Therefore, those rays that perceive the visible object [must] transmit something to the eye, by means of which the eye perceives the object. And since the rays transmit something to the eye, by means of which the eye perceives the object, the eye does not perceive the light and color in the visible object unless something comes to the eye from the light and color in the object; and this is delivered by the rays.
Therefore, according to all possibilities, sight does not occur unless something of the visible object comes from the object, whether or not rays issue from the eye. It has already been shown that sight is achieved only if the body intermediate between the eye and the visible object is transparent, and it is not achieved if the medium between them is opaque. And it is evident that a transparent and an opaque body are distinguished only in the aforesaid way. Since, as we have said and as has been demonstrated, the forms of the light and color in the visible object reach the eye (if they were [originally] opposite the eye), that which comes from the visible object to the eye (through which the eye perceives the light and color in the visible object no matter what the situation [with respect to visual rays]) is merely that form, whether or not rays issue [from the eye]. Furthermore, it has been shown that the forms of light and color are always generated in air and in all transparent bodies and are always extended to the opposite regions, whether or not the eye is present. Therefore, the egress of rays [from the eye] is superfluous and useless. Consequently, the eye does not perceive the light and color of the visible object unless form comes from the light and color [to the eye] ... .
It remains only to consider the view of those who maintain that rays issue from the eye and to indicate what in this view is false and what true. Therefore, let us assert that if sight is due to something issuing from the eye to the visible object, that thing is either corporeal or incorporeal. If it is corporeal, it follows that when we look at the sky and see the stars in it, corporeal substance issues from our eye in that time and fills the whole space between heaven and earth, and [yet] the eye is in no way destroyed; and this is [obviously] false. Therefore, sight does not occur through the passage of corporeal substance from the eye to the visible object. But if that which issues from the eye is incorporeal, it will not perceive the object, since there is no perception except in corporeal things. Therefore, nothing issues from the eye to the visible object to perceive that object. Now, it is evident that sight occurs through the eye; and since this is so, and the eye perceives the visible object only when something issues from the eye to the visible object, and since that which issues forth does not perceive the object, therefore that which issues from the eye to the visible object does not return anything to the eye, by which the eye perceives the object. And [therefore,] that which issues from the eye is not sensible but conjectural, and nothing ought to be believed except through reason or by sight.
However, those who assume that rays issue from the eye suppose this because they ascertain that the eye perceives the visible object and that between the eye and the object is space; and it is known by mankind that there is no perception except by contact. Consequently, they have conjectured that sight does not occur unless something issues from the eye to the visible object, so that the thing issuing forth perceives the object in place of the eye, or rather receives something from the visible object and returns it to the eye; then the eye perceives that thing. And since corporeal substance cannot issue from the eye to perceive the object and nothing perceives a visible object unless it is corporeal, nothing remains except to conjecture that the thing issuing from the eye to the object of vision receives something from the object and returns it to the eye. And since it has been demonstrated that air and transparent bodies receive the form of the visible object and transmit it to the eye and to every facing body, that which they conjecture to return something from the visible object to the eye is nothing but air and the transparent bodies between the eye and the object of vision. Since air and transparent bodies transmit something from the visible object to the eye, regardless of the time and according to all arrangements (when the eye is opposite the visible object), without requiring that something issue from the eye, the reasoning that led from positing rays to maintaining their existence is unnecessary; for that which led them to say that rays exist is their supposition that sight cannot be completed except through something extended from the eye to the visible object, which returns something from the object to the eye. And since air and transparent bodies do this without requiring that something issue from the eye, and moreover, since the air and transparent bodies are extended between the eye and the visible object without defect, it is useless to suppose that something else returns something from the object of vision to the eye. Therefore, it is useless to say that [visual] rays exist.
Moreover, all mathematicians who say there are rays use nothing in their demonstrations except imaginary lines, and they call them radial lines; ... and the belief of those who consider radial lines to be imaginary is true, and the belief of those who suppose that something [really] issues from the eye is false.
Translated by David C. Lindberg
Reading and Discussion Question
1.What theory of perception is Ibn al-Haytham critiquing and what alternative does he espouse?