how is it exactly that private property of means of production (the
relations of production) and a socialized production (the productive
forces) through the development of capitalism and an increasingly
large-scale industry, constitute a contradiction, and the main one, in
capitalist mode of production ?
If the property of means of production is private, and production is essentially production of commodities, then production is led by competition. Competition forces each private producer of commodities to seek to offer their commodities at the lowest price. This in turn makes each producer of commodities seek to systematically increase surplus value, which, barred the primitive and inefficient method of reducing wages and extending labour hours, is only possible through increased productivity of labour, which is only possible through increasing the composition of capital - ie, by improving the means of production so that workers produce more per unit of time.
Problem is, value is a function of labour time. So while the individual producer, when succeeding to increase the productivity of his/her unit of production, makes superprofits as long as the competition remains at an archaic level of productivity, the effect of the increase of productivity, as it spreads among all the productive system (or as it leads the least productive producers into bankrupcy), is to lower the value of each commodity, as it embodies lesser and lesser amounts of human labour.
In philosophical terms, this means that there is a contradiction between value and wealth; while value is the form of most wealth in a capitalist society, its relation to its substance, wealth, is doublefold:
- Synchronically, value is proportional to wealth: two coats are the double wealth as one coat, and the value of two coats is the double of the value of one coat;
- Diachronically, value is proportional only to labour time; doubling the wealth by halving the labour time to produce it doesn't double the value. When you are able to produce two coats in the time you formerly produced only one, wealth is doubled, but value remains the same.
At some point in the development of such a system, increasing the productivity is no longer worth the pain: gains of productivity will no longer result in increased profits. If the proportion between dead labour and live labour in a low-tech environment is 1/1, then profits will expand quite nicely by turning that proportion to 2/1. But when the proportion is already 100/1, increases in it will have very little effect on profits; the system no longer rewards gains of productivity - and has therefore come to be "an obstacle to the development of the productive forces" as you put it.
how does that contradiction manifests
itself ? Through economic crises, i've also read "the destruction of
productive forces during those crises", but how are economic crises
manifestations of this contradiction ?
Economic crises in capitalism are basically crises of overproduction. As each producer of commodities seeks to offer the lowest possible prices, and to sell the largest possible amount of individual commodities in order to compensate for lower prices, time comes when there are too many commodities in the market, and part of them becomes unsalable. Part of these commodities being productive forces themselves, if they are not sold and subsequently used, they are wasted, which is in itself a form of "destruction of productive forces".
Economic crises tend to unleash political crises, in which further destruction of productive forces may happen, in the form of wars, sabotage, etc.
how, according to marxism, is this supposed to lead to a socialist
mode of production ? Taking from Lenin and his book on imperialism,
that contradiction is being "solved" by ever-increasing concentration
of capital, leading to monopolies in different branches of production,
and the merger of branches of production, for example financial
capital, being the merger of bank capital and industrial capital. And
one can see how one large monopolistic company in one branch can
purchase or merge with another monopolistic company of another branch.
How is socialism supposed to happen or being brought ?
The short answer is it doesn't. Socialism is thought to be only possible via a political revolution, which is a deliberate action by human agents. What the development of the capitalist system provide are merely conditions for such a revolution, which may be summarised as,
- Suppression of scarcity: under previous modes of production, the amount of wealth generated by human labour was always unsufficient to provide a rich, meaningful life for all mankind if it was equitatively divided;
- Destruction of petty, "idylic" previous relations that tied human beings to their country, province, town or hamlet;
- Creation of the means for social control of production, via automation of decision making;
- Creation of a mass of individuals who are denied access to most of the wealth created, although being responsible for the creation of all value, which is thought to provide the "human agents" required for the deliberate action of revolution.
The crises of capitalism, on the other hand, provide the empirical evidence that the system is unable to work smoothly, and that the "fixes" it needs to reboot itself into a new cycle of prosperity are too costly in terms of human lives and suffering to be continuously tolerated for an indeterminate lapse of time; and also the political fractures that may make the deliberate action of revolution possible.