_B._ I say then, that in the first place you are to enquire diligently into the nature of motion. For the variations of fancies, or (which is the same thing) of the phenomena of nature, have all of them one universal efficient cause, namely the variety of motion. For if all things in the world were absolutely at rest, there could be no variety of fancy; but living creatures would be without sense of all objects, which is little less than to be dead. _A._ What if a child new taken from the womb should with open eyes be exposed to the azure sky, do not you think it would have some sense of the light, but that all would seem unto him darkness? _B._ Truly, if he had no memory of any thing formerly seen, or by any other sense perceived, (which is my supposition), I think he would be in the dark. For darkness is darkness, whether it be black or blue, to him that cannot distinguish. _A._ Howsoever that be, it is evident enough that whatsoever worketh is moved: for action is motion. _B._ Having well considered the nature of motion, you must thence take your principles for the foundation and beginning of your enquiry. _A._ As how? _B._ Explain as fully and as briefly as you can what you constantly mean by motion; which will save yourself as well as others from being seduced by equivocation. _A._ Then I say, motion is nothing but change of place for all the effect of a body upon the organs of our senses is nothing but fancy. Therefore we can fancy nothing from seeing it moved, but change of place. _B._ It is right. But you must then tell me also what you understand by place: for all men are not yet agreed on that. _A._ Well then; seeing we fancy a body, we cannot but fancy it somewhere. And therefore I think place is the fancy of here or there. _B._ That is not enough. Here and there are not understood by any but yourself, except you point towards it. But pointing is no part of a definition. Besides, though it help him to find the place, it will never bring him to it. _A._ But seeing sense is fancy, when we fancy a body, we fancy also the figure of it, and the space it fills up. And then I may define place to be the precise space within which the body is contained. For space is also part of the image we have of the object seen. _B._ And how define you time? _A._ As place is to a body, so, I think, is time to the motion of it; and consequently I take time to be our fancy or image of the motion. But is there any necessity of so much niceness? _B._ Yes. The want of it is the greatest, if not the only, cause of all the discord amongst philosophers, as may easily be perceived by their abusing and confounding the names of things that differ in their nature; as you shall see when there is occasion to recite some of the tenets of divers philosophers. _A._ I will avoid equivocation as much as I can. And for the nature of motion, I suppose I understand it by the definition. What is next to be done?