The capitalist is not forced to buy labour-power on a continuous basis. He does it only if it is profitable to him. If not, he prefers to wait, to lay off workers, or even close his plant until better times. The worker, on the other hand (the word is used here in the social meaning made clear precisely by this sentence, and not necessarily in the stricter sense of manual labourer), is under economic compulsion to sell his labour-power. As he has no access to the means of production, including land, as he has no access to any large-scale free stock of food, and as he has no reserves of money which enable him to survive for any length of time while doing nothing, he must sell his labour-power to the capitalist on a continual basis and at the current rate. Without such institutionalised compulsion, a fully developed capitalist society would be impossible. Indeed, once such compulsion is absent (for example where large tracts of free land subsist), capitalism will remain dwarfed until, by hook or by crook, the bourgeois class suppresses access to that free land.

- Ernest Mandel’s introduction to Capital, Volume 1 by Karl Marx.