Le Monde by René Descartes
Chapter 1: On the difference between our sensations and the things that produce them.
In putting forward an account of light, the first thing that I want to draw to your attention is that it is possible for there to be a difference between the sensation that we have of it, that is, the idea that we form of it in our imagination through the intermediary of our eyes, and what it is in the objects that produces the sensation in us, that is, what it is in the flame or in the Sun that we term ‘light’. For although everyone is commonly convinced that the ideas that we have in our thought are completely like the objects from which they proceed, I know of no compelling argument for this. Quite the contrary, I know of many observations which cast doubt upon it.
As you know, the fact that words bear no resemblance to the things they signify does not prevent them from causing us to conceive of those things, often without our paying attention to the sounds of the words or to their syllables. Thus it can turn out that, having heard something and understood its meaning perfectly well, we might not be able to say in what language it was uttered. Now if words, which signify something only through human convention, are sufficient to make us think of things to which they bear no resemblance, why could not Nature also have established some sign which would make us have a sensation of light, even if that sign had in it nothing that resembled this sensation? And is it not thus that Nature has established laughter and tears, to make us read joy and sorrow on the face of men?
But perhaps you will say that our ears really only cause in us sensory awareness of the sound of the words, and our eyes only sensory awareness of the countenance of the person laughing or crying, and that it is our mind which, having remembered what those words and that countenance signify, represents this to us at the same time. I could reply to this that, by the same token, it is our mind that represents to us the idea of light each time the action that signifies it touches our eye. But rather than waste time arguing, it is better to give another example.
Do you think that, when we attend solely to the sound of words without attending to their signification, the idea of that sound which is formed in our thought is at all like the object that is the cause of it? A man opens his mouth, moves his tongue, and breathes out: I see nothing in all these actions which is in any way similar to the idea of the sound that they cause us to imagine. And most philosophers maintain that sound is only a certain vibration of the air striking our ears. Thus if the sense of hearing transmitted to our thought the true image of its object, then instead of making us think of the sound, it would have to make us think about the motion of the parts of the air that are vibrating against our ears. But as not everyone will, perhaps, wish to follow what the Philosophers say, so I shall offer another example.
Of all our senses, touch is the one considered least deceptive and the most secure; so if I show you that even touch leads us to conceive many ideas which do not resemble in any way the objects that produce them, I believe you should not find it strange when I say that the same holds for sight. Now everyone knows that the ideas of tickling and pain which are formed in our thought when bodies from outside touch us bear no resemblance at all to these. One passes a feather lightly over the lips of a child who is falling asleep and he feels himself being tickled: do you think that the idea of tickling which he conceives resembles something in the feather? A soldier returns from battle. During the heat of the combat he could have been wounded without being aware of it. But now, as he begins to cool down he feels pain and believes that he has been wounded: a surgeon is called and examines him once his armour has been removed; in the end, it is discovered that what he was feeling was just a buckle or strap which, being caught under his armour, was pressing on him and causing his discomfort. If his sense of touch, in causing him to feel this strap, had impressed its image in his thought, there would not have been any need for the surgeon to show him what he was feeling.