Marx claims that no previous theorist has been able adequately to explain how capitalism as a whole can make a profit. Marx’s own solution relies on the idea of exploitation of the worker. [...] The first four hours of the working day is spent on producing value equivalent to the value of the wages the worker will be paid. This is known as necessary labour. Any work the worker does above this is known as surplus labour, producing surplus value for the capitalist.
Surplus value, according to Marx, is the source of all profit.
In Marx’s analysis labour power is the only commodity which can produce more value than it is worth, and for this reason it is known as variable capital. Other commodities simply pass their value on to the finished commodities, but do not create any extra value. They are known as constant capital. Profit, then, is the result of the labour performed by the worker beyond that necessary to create the value of his or her wages. This is the surplus value theory of profit.
It appears to follow from this analysis that as industry becomes more mechanised, using more constant capital and less variable capital, the rate of profit ought to fall. For as a proportion less capital will be advanced on labour, and only labour can create value.
In Capital Volume 3 Marx does indeed make the prediction that the rate of profit will fall over time, and this is one of the factors which leads to the downfall of capitalism. [...]
Marx’s assertion that only labour can create surplus value is unsupported by any argument or analysis, and can be argued to be merely an artifact of the nature of his presentation. Any commodity can be picked to play a similar role. Consequently with equal justification one could set out a corn theory of value, arguing that corn has the unique power of creating more value than it costs. Formally this would be identical to the labour theory of value.