they better define it than by motion?  For is it not at least as proper and significant to say, Passage is a motion from one place to another, as to say, Motion is a passage, &c.?  This is to translate, and not to define, when we change two words of the same signification one for another; which, when one is better understood than the other, may serve to discover what idea the unknown stands for; but is very far from a definition, unless we will say every English word in the dictionary is the definition of the Latin word it answers, and that motion is a definition of MOTUS.  Nor will ’the successive application of the parts of the superficies of one body to those of another,’ which the Cartesians give us, prove a much better definition of motion, when well examined.

10.  Definitions of Light.

‘The act of perspicuous, as far forth as perspicuous,’ is another Peripatetic definition of a simple idea; which, though not more absurd than the former of motion, yet betrays its uselessness and insignificancy more plainly; because experience will easily convince any one that it cannot make the meaning of the word light (which it pretends to define) at all understood by a blind man, but the definition of motion appears not at first sight so useless, because it escapes this way of trial.  For this simple idea, entering by the touch as well as sight, it is impossible to show an example of any one who has no other way to get the idea of motion, but barely by the definition of that name.  Those who tell us that light is a great number of little globules, striking briskly on the bottom of the eye, speak more intelligibly than the Schools:  but yet these words never so well understood would make the idea the word light stands for no more known to a man that understands it not before, than if one should tell him that light was nothing but a company of little tennis-balls, which fairies all day long struck with rackets against some men’s foreheads, whilst they passed by others.  For granting this explication of the thing to be true, yet the idea of the cause of light, if we had it never so exact, would no more give us the idea of light itself, as it is such a particular perception in us, than the idea of the figure and motion of a sharp piece of steel would give us the idea of that pain which it is able to cause in us.  For the cause of any sensation, and the sensation itself, in all the simple ideas of one sense, are two ideas; and two ideas so different and distant one from another, that no two can be more so.  And therefore, should Des Cartes’s globules strike never so long on the retina of a man who was blind by a gutta serena, he would thereby never have any idea of light, or anything approaching it, though he understood never so well what little globules were, and what striking on another body was.  And therefore the Cartesians very well distinguish between that light which is the cause of that sensation in us, and the idea which is produced in us by it, and is that which is properly light.