It stimulates the labourers to demand more than they receive already, and it stimulates to demand the more on the ground that they themselves have produced it.

It teaches them that the wealth of every man who is not a manual labourer is something stolen from themselves which ought to be and which can be restored to them.
Now, whatever may be the value of such teaching as a contribution to economic science, it illustrates by its success one cardinal truth, and by implication it bears witness to another.

The first truth is that, no matter how desirable any object may be which is obtruded on the imagination of anybody, nobody will bestir himself in a practical way to demand it until he can be persuaded to believe that its attainment is practically possible.

The other is this: that the possibilities of redistributing wealth depend on the causes by which wealth is produced.
All wealth, says Marx, can practically be appropriated by the labourers.

But why?
Because the labourers themselves comprise in their own labour all the forces that produce it.