Socialised capital is capital which does not exist in the form of private property. If we take the capital of any small producer, such as a peasant producer, all of this capital takes the form of private property. It does not matter whether this capital is fixed capital or circulating capital, constant capital or variable capital, it belongs to the individual capitalist owner, and can be used by them as they choose. That is not the case with socialised capital.
A small capitalist producer can use a building in their possession for any purpose they choose, from carrying on production to holding a private party for their family and friends; they can use any of the machines in their workshop to produce commodities for sale, or to produce items for consumption by themselves, family and friends; they can use any of the materials in their possession for productive purposes, or for their own consumption; they can take any of the commodities that the business produces, and which constitutes its commodity-capital, and consume them themselves or in any way they choose.
This is true whether this capital is the private property of a sole trader, a family business, or even a large business that is privately owned by an individual or family. But, there are obvious limitations to such forms of private capital. As Marx puts it in Capital Volume I,
“The monopoly of capital becomes a fetter upon the mode of production, which has sprung up and flourished along with, and under it. Centralization of the means of production and socialization of labour at last reach a point where they become incompatible with their capitalist integument. This integument is burst asunder. The knell of capitalist private property sounds. The expropriators are expropriated.” (p 714-5)
This expropriation of capitalist private property occurs in the form of the rise of socialised capital, in the shape of joint stock companies, and co-operatives. The socialised capital refers to the actual capital of the business. That is the money-capital in its bank account and cash box, the productive-capital in the shape of buildings, machines, materials and labour-power, and the commodity-capital in the shape of the finished products waiting to be sold. All of this capital is the property of the business, as a corporate entity in its own right, and not to any particular individual or group of individuals.
Unlike capital in the form of private property, no individual or group of individuals owns this capital, and so has no right to use it for their own purposes. That right now belongs to the firm itself. This is set down clearly in English Law.
"A company is an entity distinct alike from its shareholders and its directors.” (Shaw & Sons (Salford) Ltd v Shaw  2 KB 113 by Greer LJ.
As Engels put it,
“Capitalist production by joint-stock companies is no longer private production but production on behalf of many associated people. And when we pass on from joint-stock companies to trusts, which dominate and monopolise whole branches of industry, this puts an end not only to private production but also to planlessness.”
Or as Marx put it,
“The capital, which in itself rests on a social mode of production and presupposes a social concentration of means of production and labour-power, is here directly endowed with the form of social capital (capital of directly associated individuals) as distinct from private capital, and its undertakings assume the form of social undertakings as distinct from private undertakings. It is the abolition of capital as private property within the framework of capitalist production itself...
It is the point of departure for the capitalist mode of production; its accomplishment is the goal of this production. In the last instance, it aims at the expropriation of the means of production from all individuals. With the development of social production the means of production cease to be means of private production and products of private production, and can thereafter be only means of production in the hands of associated producers, i.e., the latter's social property, much as they are their social products.”
(Capital III, Chapter 27)
This form of capital as socialised capital, rather than as capitalist private property, arises naturally from the very laws and dynamic of capitalist production, and the development of the means of production as social means of production.
“The contradiction between the general social power into which capital develops, on the one hand, and the private power of the individual capitalists over these social conditions of production, on the other, becomes ever more irreconcilable, and yet contains the solution of the problem, because it implies at the same time the transformation of the conditions of production into general, common, social, conditions. This transformation stems from the development of the productive forces under capitalist production, and from the ways and means by which this development takes place.”
(Capital III, Chapter 15)
The socialised capital develops as the only logical means upon which accumulation can proceed on an adequate basis, given the large scale upon which production takes place. The different forms that this socialised capital assumes do not reflect the actual ownership of the capital itself, but different forms of corporate governance, and so control over that capital. Whether the socialised capital takes the form of a worker owned co-operative, a consumer co-operative, a public limited company, a trust, or a nationalised industry, the capital in each case, itself is owned by the company.
This is important for the reasons Marx sets out in Part V, of Capital Volume III. Capital produces profit, the phenomenal form of surplus value under capitalism. But, in order to function and produce profit, a firm must first acquire the use of that capital, as well as acquire land upon which to undertake production. Where a firm borrows capital, what it actually does, as Marx sets out, is to buy the use value of capital to produce profit. In other words it buys capital itself as a commodity. This is different to buying the commodities that comprise constant capital, or variable capital. When a firm buys a machine, that subsequently forms part of its constant capital, it is not buying capital, but only a machine as a commodity.
A machine has a value as a commodity, like every other commodity. But, what makes this machine capital is the subsequent historical and social context in which it functions. That is to produce profit, to be self-expanding value. That is its use value, its nature as capital, as opposed to its use value simply as a machine. But, this specific use value of capital, of being self expanding value, in itself has no value, because it is not the product of labour. The price of capital, therefore, as a commodity – the rate of interest – is not determined by the labour-time required for its production, nor by its price of production, because neither of these things apply. It is determined solely by the demand for capital, and the supply of capital.
Forward To Part 2