This truth can be yet further elucidated by the very illustration which Mill cities in disproof of it. It is absurd to ask, he says, whether the number five or six does most, when they are multiplied together, to produce "the effect" thirty. This is true so long as "the effect" thirty is constant; but if on occasions the thirty is increased to forty, and if whenever this happens the six has increased to eight, we know that the extra ten which our multiplication yields us is not due to the five, the number which remains unchanged, but to an extra two now present in the number that was once six. Or again let us take as "the effect" the speed of a motor-car which is raced over a mile of road. Unless two conditions were present--the engine and some ground to run upon--the car could not run at all; and if there were only one road and one car in the world, it would be absurd to inquire how much of the speed was due to the merits of the engine, and how much to the character of the road's surface. But if, the car remaining unchanged, the surface of the road was improved, and a speed was thereupon developed of thirty miles an hour instead of twenty, we should, with regard to the increment, at once be able to say that it was due to the surface of the road, and was not due to the engine. Conversely, if the road were unchanged, but the car had a new engine, and the speed under these conditions increased in the same way, the increment would be evidently attributable to the engine and not the road.