From these axioms we can deduce that a set of points which is -collinear is collinear. Also that, given a set of -collinear points, if is one of them, the points of the set which are beyond from a are -collinear, and retain the same order when arranged with reference to as they had when arranged with reference to . Also that, if is one of a set of -collinear points, those of the set which are between and are -collinear, and have, when arranged with reference to , the converse order to that which they had when arranged with reference to . These propositions show that we have a satisfactory definition of order among the points of a collinear set. The above axioms are logically adequate, but regarded as asserting physical truths about events they may perhaps be regarded as more or less doubtful. We have to remember that our lines are not straight, and may therefore return into themselves. Routes with very great curvature are, however, excluded by our definition of collinearity. Consider, e.g., such a route as that in the accompanying figure. We may suppose that , , , are all connected, but and will not be between and according to the definition, because obviously an event may contain and without containing and . Thus if we wish to regard the above route from to as, in some sense, a line, it will have to be in an extended sense, namely, that it can be divided into a number of small finite parts, each of which is a line. And a set of points may be regarded as collinear in an extended sense if it is capable[Pg 309] of a serial order such that any sufficiently small consecutive stretch of the series is a collinear set—provided that such stretch must contain not less than four points. We can now prove, by the help of one further axiom, that any progression of collinear points all lying between two points and must have a limit. Let our set of points be all lying on a line between and , in an order from towards . Let be the sum of all the points in (i.e. the class of members of members of ), and their product, i.e. the events which belong to every member of . Then is not null, because is contained in it, and , are connected (in virtue of the definition of collinearity). Let consist of all the 's except , of all the except , etc. Let be the events belonging to all members of and generally let be the events belonging to all members of ; and let be the sum of all the 's. Then consists of all those events which belong to all sufficiently late 's; i.e. to say that an event is a member of is to say that there is an such that the event is a member of for all values of . It will be observed that is contained in , therefore is contained in . It follows that, if , are two members of , there is an such that , are both members of . Hence they are both members of . Hence any five members of are co-punctual, and therefore there is at least one point which contains the whole of , since is contained in .