It’s best to start with an excerpt from a classic text:
One of the greatest injustices and one contributing most to the destruction of civilisation is the unjustified imposition of tasks and the use of the subjects for forced labour. This is so because labour belongs to the things that constitute capital. Gain and sustenance represent the value realised from labour among civilised people. By their efforts and all their labours they (acquire) capital and (make a) profit. They have no other way to make a profit except (through labour). Subjects employed in cultural enterprises gain their livelihoods and profit from such activities. Now, if they are obliged to work outside their own field and are used for forced labour unrelated to their (ordinary ways of) making a living, they no longer have any profit and are thus deprived of the price of their labour, which is their capital (asset). They suffer, and a good deal of their livelihood is gone, or even all of it.
If you think this is something out of Das Kapital by Karl Marx, try going back about five hundred years to North Africa. It is taken from the magisterial Muqaddimah (Prolegomena) by Ibn Khaldun, the renowned fourteenth century Arab scholar (d. 1406). The Muqaddimah covers much more than economics – it is no less than the author’s self-proclaimed ‘introduction to history’. Impressively, it contains insights that appear ahead of its time, including passages, such as the one quoted above, that seem to pre-figure Marx’s theory of wage-labour and capital.