Chris has sent me an e-mail message asking for my views on Marx and justice, with particular attention to the question why Marx thinks that exploitation is unjust. Since this may be of more general interest, I have decided to reply here rather than in a private response. The simple answer to Chris's question is that in my view, Marx did not think that exploitation is unjust, but you had better settle down, because it is going to take me a while to spell this out fully.
Marx considers the philosophy of a society to be a part of its ideological superstructure, along with its religion, law, and art, among other things. Moral judgments are a part of the philosophy and law of a society, hence ideological and superstructural as well. The fundamental principle of bourgeois justice is that equals be given for equals in a free and open marketplace where men [it is always men] meet one another as legal equals, none compelled by law or custom to enter into bargains with another. The ideal capitalist, Marx argues, pays a fair price for the labor he employs. He pays a price equivalent to the reproduction cost of that labor, which, as he and Ricardo would say, is equal to the labor value embodied in that labor. Now, to be sure, capitalists do not play fair. As Marx tells us in the great chapter on The Working Day, capitalists try such underhanded tricks, in their effort to extract more value from their workers, as fiddling with the clocks in the factory so as to make the workers labor for a bit longer than the contracted for ten or twelve hours. But this is not exploitation. This is just cheating.
Exploitation is the extraction from a factor input of more value than is contained within it. A good deal of Volume One is devoted to discovering how capitalists manage to pull off this trick -- the secret to profit. Marx's solution -- which, as I have explained at length elsewhere is in my opinion incorrect -- is the distinction between labor and labor power.
But what would socialism have to say about exploitation? Is it not the case that exploitation is, from a socialist perspective, unjust? This, Marx thinks, is a fundamentally confused question. It is as confused as asking whether, from a bourgeois perspective, feudal laws regulating the making and selling of craft goods are unjust. Exploitation would indeed be unjust in a socialist society, just as exploitation is just in a bourgeois society.
But from a transhistorical rational point of view, which one is correct -- socialist morality or bourgeois morality? That, Marx thinks, is a meaningless question. There is no pou sto, no place to stand from which one can make objective, neutral moral judgments uninfected with the ideological perspective of any particular society. That is precisely the fatal illusion of such covertly bourgeois ideological rationalizations as the "theory of justice" of John Rawls.
This may sound plausible, but surely it is wrong. Marx is no bloodless observer of social reality, reporting what he finds without judgment, in the manner affected [self-deceivingly] by modern sociologists and economists. No one has ever thundered more powerfully against bourgeois injustice than Marx! Quite true, quite true. But Marx is not a moralist.
A descent into armchair psychoanalysis is called for here. In Victorian England there was a popular parlor game called Confessions. Each person in the gathering would be asked to state his or her favorites: favorite color, favorite food, favorite author, favorite literary character. There are, in the Marx's collected papers, several sheets listing Marx's responses to the game of Confessions. His favorite trait in men? Strength. His favorite trait in women? Weakness. [I have this from the great biography, Marx's Fate, by Jerrold Siegel. You see perhaps why I do not like Marx very much as a human being, for all that I consider him the greatest social scientist who has ever lived.] Marx hated weakness in men, and considered the making of moral judgments as the last resort of the weak. To say "That is morally wrong" is implicitly to say, "I am powerless to stop that, so I shall inveigh against it." [If this makes you think of Nietzsche, you would be right.]
Marx expressed his deep loathing of capitalism not by offering moral condemnations of it -- troubling deaf heaven with his bootless cries, as it were -- but by proving rigorously, scientifically, irrefutably, that it was doomed to self-destruct.
So, Chris, my response to your question is this: Marx does not offer an argument that exploitation is morally unjust. He offers a static scientific analysis of exploitation as essential to capitalism, and a dynamic scientific argument that capitalist exploitation will be replaced with socialism, in which exploitation will play no role.
And will socialism then be a just social and economic order? Marx does not say. There are two answers that might be inferred from his analysis of capitalism. If you think that in a socialist society there will be no ideological superstructure, because all mystification will have been dispelled by revolution, then the answer is that a socialist society will have no need for such ideological apparatuses as moral theory, so the question will be moot. But if you think that even in a socialist society there will be an ideological superstructure, then you can be certain that from a socialist perspective, socialism will be just, as from a bourgeois perspective capitalism is just.