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.
And the same observations apply to labour and directive ability, whenever the operations of both are essential to a given product.

If the ability and the labour were always inevitably constant, and the product as to quality and amount were similarly constant also, we could not say that so much or so little of the effect was due to one cause, and so much or so little to the other.

If there were in the world only a thousand shipwrights, and these men, working always under the same director, always produced in a year one ship of an unchanging kind, we could not say which of its parts or how much of its value were due to the man directing, and which or how much were due to the men directed.
But if for one year this director were to retire and another was to take his place, and, the same labourers being directed by this new master, the result was the production not of one ship but of two; and if, when the year was ended, and the old master came back again, the annual product once more was not the two ships but one, we could then say, as a matter of common-sense with regard to the year during which the two vessels were built, that the second vessel, whatever might be the case with the first, was due wholly to the ability of the master, and not to the labour of the men.

In other words, the ability of the director of labour produces so much of the product, or of that product's value as exceeds what was produced by the labourers before their labour was directed by him, and would cease to be produced any longer as soon as his direction was withdrawn.
That in the case of any result which requires separable causes for its production, this method of allocating to these causes respectively so much of the result and so much of it only, is a method always adopted in all practical reasoning, may be seen by taking a result which is not beneficial but criminal.