senses as a measure of the things they can know.

[But it seems to me to be doing great wrong to human reason if we do not consider that knowledge goes beyond the seen]; for no one can doubt that there are bodies so small that they cannot be perceived by any of our senses, if only we consider what is being added each moment to those bodies which increase little by little, and what is removed from those which diminish in the same fashion. We day by day see a tree grow, and it is impossible to comprehend how it becomes larger than it was before, unless by conceiving that some body is added to it. But who has ever observed by means of the senses what are the small bodies which are each day added to the plant that grows? Those at least who hold quantity to be finitely divisible should acknowledge that the particles may become so small as to be absolutely imperceptible. And indeed it should not be wondered at that we are unable to perceive very minute bodies, for the nerves, which must be moved by objects in order to cause us to perceive, are not very minute, but are like small cords which consist of a quantity of yet smaller fibres, and thus they cannot be moved by the minutest of bodies. Nor do I think that anyone who uses his reason will deny that we do much better to judge of what takes place in small bodies which their minuteness alone prevents us from perceiving, by what we see occurring in those that we do perceive [and thus explain all that is in nature, as I have tried to do in this treatise], than, in order to explain certain given things, to invent all sorts of novelties, that have no relation to those that we perceive [such as are first matter, substantial forms, and all the great array of qualities which many are in the habit of assuming, any of which it is more difficult to understand than all the things which we profess to explain by their means].

PRINCIPLE CCVII

Nevertheless all my opinions are submitted to the authority of the church.

At the same time, recalling my insignificance, I affirm nothing, but submit all these opinions to the authority of the Catholic Church, and to the judgment of the more sage; and I wish no one to believe anything I have written, unless he is personally persuaded by the force and evidence of reason.

SELECTIONS FROM THE WORLD; OR ESSAY

ON LIGHT

CHAPTER I

Of the difference between our sensations and the things which produce them

Proposing, as I do, to treat of the nature of light, the first thing of which I wish you to take note is, that there may be a difference between the sensation which we have in ourselves, that is to say, the idea which is formed within our imagination by the help of our eyes, and that which exists in the objects that produce within us the sensation, namely, that which exists in the flame, or in the sun, and is called by the name of light; because, although everyone is commonly persuaded that the ideas that we have in our thought are altogether similar to the objects whence they proceed, I see no reason, nevertheless, to assure us that this is true; but, on the contrary, I observe many facts which should incline us to question it.

You know that words, while having no resemblance to the things which they signify, do not fail to make them intelligible to us, and often, even without our paying attention to the sound of the words, or to their syllables; so that it may happen that after having listened to a discourse, the meaning of which we have completely understood, we are not able to say in what language it was spoken. But if words, which signify nothing except by human institution, are capable of making conceivable for us things to which they have no

resemblance, why may not nature also have established a certain sign which should make us feel the sensation of light, although this sign should have nothing in itself resembling sensation? Has she not thus appointed laughter and tears to make us read joy and sadness in the human countenance?

But you will say, perhaps, that our ears make us perceive in reality merely the sound of the words, and our eyes only the face of him who laughs or who weeps, and that it is our mind, which, having retained what these words and this countenance signify, represents it to us at the same time. To that I may reply that, just in the same way, it is our mind which represents to us the idea of light whenever the action which signifies it touches our eye; but, without wasting time in dispute, I will at once bring forward another illustration.

Do you think that when we pay no attention to the meaning of words, and only hear the sound of them, that the idea of this sound, which is formed within our thought, is anything like the object which is the cause of it? A man opens his mouth, moves his tongue, expels his breath; I see nothing in all these motions which is not quite different from the idea of the sound which they cause us to imagine. And most philosophers assure us that the sound is nothing but a certain trembling of the air which has just struck our ears; so that, if the sense of hearing brought to our thought the true image of its object, it would be necessary, in place of making us conceive the sound, that it should make us conceive the motion of the portions of the air which is trembling at the time against our ears. But because, perhaps, everybody will not believe what the philosophers say, I will adduce still another example. Touch is the one of all the senses which we consider the least deceptive and the most trustworthy; so that, if I prove

to you that even touch makes us conceive many ideas which do not at all resemble the objects which produce them, I do not think you ought to consider it strange if I say that sight may do the same.

But there is no one who does not know that the ideas of pleasure and of pain which are formed within our thought on occasion of bodies touching us externally have no resemblance to them. A person gently passes a feather over the lips of a child asleep, and he perceives the tickling; do you suppose that the idea of the tickling which he conceives has any resemblance to anything there is in the feather? A soldier returns from a fight; during the heat of the combat he might have been wounded without perceiving it, but now that he begins to cool off he feels pain, he thinks he has been wounded; a surgeon is called, his uniform is stripped off, he is examined, and at last it is found that what he felt was nothing but a buckle or a strap, which, being twisted underneath his uniform, pressed upon him and hurt him. If his sense of touch, while making him feel the strap, had impressed the image of it on his thought, he would not have needed a surgeon to tell him what he felt.

But I see no reason which obliges us to think that what is in the objects from which the sensation of light comes to us is any more like that sensation than the action of a feather and a buckle is like the tickling and the pain; and yet I have not adduced these examples in order to make you believe absolutely that this light is something different in the objects from what it is in our eyes, but simply that you may question it, and that, being on your guard against a prejudice to the contrary, you may now the better inquire with me into the true state of the case.