When, with any practical purpose in view, we insist that anything is the cause of anything else, or produces anything else, we are always selecting, out of an incalculable number of causes, one cause or agency which, under the circumstances in view, may or may not be present; which a careless person may neglect to introduce; which an ignorant person may be persuaded to take away; or a recognition of which will influence human conduct somehow; while all other causes, which no one proposes to take away, or which no one is able to take away, are assumed by all parties, but they are not considered by anybody. Why should they be considered? Not only are they so numerous that no intellect could deal with them, but they have, since with regard to them there is no difference of opinion, no place in any practical discussion at all. If a ton of stone is to be placed on a piece of framework, men may reasonably discuss whether the framework is strong enough to bear it, or whether material is not being wasted in making it stronger than necessary. What will happen without an additional girder? Or what will happen if we take two girders away? Will the stone fall or not? These questions belong to the domain of practical reasoning because to take a girder away, or else introduce fresh ones, lies within the power of the disputants. But no practical men would think of complicating the discussion by calculating what would happen if they suspended the action of gravitation, in which case the stone would need no support whatever; for to suspend the action of gravitation is within the power of nobody. If two men are debating in the middle of the night at midsummer whether there is enough oil in the lamp to keep it alight till sunrise, they are debating a question of a strictly practical kind: for it rests with them to put in more oil or not. What will happen if they do not? That is the point at issue. But they neither of them would debate what would happen if the movement of the earth were retarded, and the midsummer morning were delayed till the hour at which it dawns in winter. They do not discuss this contingency, for they rightly assume it to be impossible, and consequently the discussion of it would have no practical meaning. And now let us go back to the question of labour and ability; and we shall see, in the case of products to the production of which both are essential, that, while ability is the practical cause of all such amounts or values as exceed what would have been produced by labour if there were no ability to direct it, it cannot be claimed in any similar sense that all amounts and values are conversely produced by labour, which exceed what would have been produced by the action of directive ability, if no labour existed for such ability to direct.