time *t*, it must be possible to find two instants *t*_{1}, *t*_{2}, one earlier than *t* and one later, such that throughout the whole time from *t*_{1} to *t*_{2} (both included), the particle lies between P_{1} and P_{2}. And we say that this must still hold however small we make the portion P_{1} P_{2}. When this is the case, we say that the motion is continuous at the time *t*; and when the motion is continuous at all times, we say that the motion as a whole is continuous. It is obvious that if the particle were to jump suddenly from P to some other point Q, our definition would fail for all intervals P_{1} P_{2} which were too small to include Q. Thus our definition affords an analysis of the continuity of motion, while admitting points and instants and denying infinitesimal distances in space or periods in time.

Philosophers, mostly in ignorance of the mathematician’s analysis, have adopted other and more heroic methods of dealing with the *primâ facie* difficulties of continuous motion. A typical and recent example of philosophic theories of motion is afforded by Bergson, whose views on this subject I have examined elsewhere.^{[1]}

Apart from definite arguments, there are certain feelings, rather than reasons, which stand in the way of an acceptance of the mathematical account of motion. To begin with, if a body is moving at all fast, we *see* its motion just as we see its colour. A *slow* motion, like that of the hour-hand of a watch, is only known in the way which mathematics would lead us to expect, namely by observing a change of position after a lapse of time; but, when we observe the motion of the second-hand, we do not merely see first one position and then another—we see something as directly sensible as colour. What is this something that we see, and that we call visible mo-