You know, of course, that words make us form conceptions of the things they signify even though they have no resemblance to them, often even without our paying any attention to the sounds of the words or the syllables of which they are composed. Thus it can happen that, after hearing something said of which we have perfectly understood the sense, we are unable to say what language it was spoken in. But if words, which have meaning only as a human institution, are enough to make us form conceptions of things they bear no resemblance to, why could not Nature too have instituted some sign which would make us have the sensation of light, but without containing in itself anything similar to this sensation? And is this not how she has instituted smiles and tears to make us read joy and sadness on people’s faces?

But perhaps you will say that our ears really only make us perceive the sound of the words, and our eyes the countenance of the person who smiles or weeps, and that it is our spirit which, having grasped the meanings of the words and countenance, represents them to us at the same time. I could reply to this that it is likewise our spirit which represents to us the idea of light whenever the action which signifies it comes into contact with our eyes. But rather than [5] wasting time in disputation, I would prefer to give another example.

When we ignore the meanings of words and listen only to their sound, do you think the idea of this sound formed in our thinking bears some resemblance to the object that causes it? Someone opens their mouth, moves their tongue, emits their breath — but I see nothing in all these actions that is not very different from the idea of the sound which they make us form in our imagination. And the majority of philosophers assure us that sound is nothing but a certain vibration of the air which comes and beats against our ears; so that if the sense of hearing brought the true image of its object into our thinking, instead of making us have a conception of sound, it would have to make us have a conception of the motion of the parts of the air then vibrating against our ears. But since not everyone, perhaps, will be prepared to believe what philosophers say, I shall give yet another example.

The sense which is considered the least deceptive and the most certain is that of touch; so, if I show you that even the sense of touch makes us conceive many ideas which have no resemblance at all to the objects that produce them, I do not think you should find it strange if I say that the sense of sight can do the same. There is no one who does not know that the ideas of tickling and of pain which are formed in our thinking on the occasion [6] of our coming into contact with external bodies bear no resemblance to them. You gently pass a feather over the lips of a sleeping child, and it senses that you are tickling it: do you think that the idea of tickling which it conceives resembles in any respect the qualities of the feather? A soldier returns from a battle: during the heat of the action he could have been wounded without noticing it; but now that he is beginning to cool off, he feels some pain, and believes he has been wounded. A surgeon is called, his armour is removed, the surgeon makes a visit, and finally it is found that what he felt was nothing other than a buckle or a strap which had got caught up under his armour and caused the trouble by pressing into him. If his sense of touch, in making him aware of this strap, had impressed the image of it on his thinking, he would have had no need of the surgeon to tell him what he was feeling.

So, I see no reason why we should believe that whatever it is in objects that gives rise to our sensation of light is any more like that sensation than the actions of a feather or a strap are like the sensation of tickling or pain. However, I have certainly not brought up these examples in order to make you believe absolutely that light is different in objects from what it is in our eyes; but only in order to make you reserve judgment about it; and, by keeping you from being prejudiced by the contrary opinion, to enable you to join me now in a more fruitful examination of its nature. . . . .