_A._ This is certainly true of a staff or other hard body, because it keeps the motion in a straight line from diffusion. But in such a fluid body as the air, which being put back must fill an orb, and the farther it is put back, the greater orb, the motion will decrease, and in time, by the resistance of air to air, come to an end. _B._ That any body in the world is absolutely at rest, I think not true: but I grant, that in a space filled everywhere with body, though never so fluid, if you give motion to any part thereof, that motion will by resistance of the parts moved, grow less and less, and at last cease; but if you suppose the space utterly void, and nothing in it, then whatsoever is once moved shall go on eternally: or else that which you have granted is not true, viz., that nothing can put an end to its own motion. _A._ But what mean you by resistance? _B._ Resistance is the motion of a body in a way wholly or partly contrary to the way of its movent, and thereby repelling or retarding it. As when a man runs swiftly, he shall feel the motion of the air in his face. But when two hard bodies meet, much more may you see how they abate each other’s motion, and rebound from one another. For in a space already full, the motion cannot, in an instant, be communicated through the whole depth of the body that is to be moved. _A._ What other definitions have I need of? _B._ In all motion, as in all quantity, you must take the beginning of your reckoning from the least supposed motion. And this I call the first endeavour of the movent; which endeavour, how weak soever, is also motion. For if it have no effect at all, neither will it do anything though doubled, trebled, or by what number soever multiplied: for nothing, though multiplied, is still nothing. Other axioms and definitions we will take in, as we need them, by the way. _A._ Is this all the preparation I am to make? _B._ No, you are to consider also the several kinds and properties of motion, viz., when a body being moved by one or more movents at once, in what way it is carried, straight, circular, or otherwise crooked; and what degree of swiftness; as also the action of the movent, whether trusion, vection, percussion, reflection, or refraction; and farther you must furnish yourself with as many experiments (which they call phenomenon) as you can. And supposing some motion for the cause of your phenomenon, try, if by evident consequence, without contradiction to any other manifest truth or experiment, you can derive the cause you seek for from your supposition. If you can, it is all that is expected, as to that one question, from philosophy. For there is no effect in nature which the Author of nature cannot bring to pass by more ways than one.