Specifically, all labor can be measured in units of abstract unskilled labor. The aggregate labor-time of society is the absolute scarce resource of society. The labor which is embodied in economic products as value can be uniquely attributed to the individuals who did the labor. The value of each person's contribution to society can be measured precisely. Thus, the amount of surplus-labor done by each individual can be measured.
Profit exists in every socio-economic formation, in the form of surplus labor.
Economic justice means that each individual producer receives a share in the means of consumption proportionate to his or her labor-contribution. Economic progress consists in the rising productivity of labor. German Ideology, p. 49. 1, Chs, I, II, esp. Capital, Vol. 2, p. 358.