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.
The reason why labour, in this respect, differs from ability is as follows: Whether directive ability shall or shall not exert itself depends upon human volitions which, according to circumstances, are alterable, just as it depends upon alterable human volitions whether a framework of steel be constructed in this way or in that; or whether a lamp be replenished with oil or no.

But whether ordinary manual labour shall or shall not exert itself, is not similarly dependent on human volition at all.

Let a nation be organised, no matter on what principles, the majority of the citizens will have to labour in any case.