The fact is that we have to play the game--we are not merely spectators--and as a matter of fact, also, men do assume that they can freely choose what moves they will make and that there are certain moves which they ought to make. The assumptions which they make, not exactly for the sake of playing the game, but in the act of playing it, are neither included in the assumptions of science nor excluded by them. To play the game at all, it is necessary to have some knowledge (or to act as though we had some knowledge) of how the pieces move, to know that bishops move diagonally, that bodies tend to gravitate at a certain rate. Man cannot indeed act or make the slightest movement without deflecting or starting some of the processes of Nature and of his own psychological mechanism: it is through them that he operates, and by means of them that he plays the game. In the beginning he has but little knowledge of what the consequences will be if he touches this or that spring of the mechanism. Yet the knowledge is necessary for him, if he is to play the game as he ought, _i.e._ to attain the moral ideals of which he is more or less (less at first) conscious. In acquiring this knowledge he uses his faculty of abstraction, that is his power of concentrating his attention on one aspect of a thing or problem, to the exclusion of the rest, in order to gain a clearer knowledge of it by giving it his undivided and undistracted attention. Thus, in order to understand how the mechanism of Nature or human nature actually does act, he concentrates his attention on the working of that mechanism in the abstract, _i.e._ wholly apart from the fact that it is at times started, at times interrupted, or redirected for the sake of realising (or thwarting) his ideals. The knowledge thus gained is science, and is, according to the agnostic, the optimist, and the pessimist, the only knowledge that man can have.