moving point must move in a certain 'direction'—that, if two points, starting from a state of coincidence, move along two equal straight Lines which do not coincide (so that their movements are alike in point of departure, and in magnitude), that quality of each movement, which makes it differ from the other, is its 'direction'—and similarly that, if two equal straight Lines are terminated at the same point, but do not coincide, that quality of each which makes it differ from the other, is its 'direction' from the common point?
Nie. It is all very true: but you are using 'straight Line' to help you in defining 'direction.' We, on the contrary, consider 'direction' as the more elementary idea of the two, and use it in defining 'straight Line.' But we clearly agree as to the meanings of both expressions.
Min. I am satisfied with that admission. Now as to the phrase 'the same direction,' which you have used in reference to a single Line and the motion of a single point. May we not say that portions of the same Line have 'the same direction' as one another? And that, if a point moves along a Line without turning back, its motion at one instant is in 'the same direction' as its motion at another instant?
Nie. Yes. That expresses our meaning in other language.
Min. I have altered the language in order to bring out clearly the fact that, in using the phrase 'the same direction,' we are really contemplating two Lines, or two motions. We have now got (considering 'straight Line' as an understood phrase) accurate geometrical Definitions
