The problem is that if everything is traded for the cost that it takes to produce it, then where would we ever get a profit? The answer, Marx gives, is that that there is one difference between HUMAN labour power as a commodity, and that of every other non-human commodities. The cost of producing a human being (i.e. it's sustenance" might not necessarily be equal to the amount of value that a worker can PRODUCE in a given day. Thus, if it takes 6 hours of labor time to provide for sustenance, then the remaining hours of work time (6 or so in Marx's day) are SURPLUS -- they are the value over-and-above costs that the worker produces.
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I guuuuuess.
@.@