In mainstream economic theory, the firm and the market are, for the most part, assumed to exist and are not themselves the subject of investigation. … - Ronald Coase

Why... are there any market transactions at all? Why not all production carried on by one big firm?... First, as a firm gets larger, there may be decreasing returns to the entrepreneur function, that is, the costs of organizing additional transactions within the firm may rise... Second, it may be that as the transactions which are organized increase, the entrepreneur fails to place the factors of production in the uses where their value is greatest, that is, fails to make the best use of the factors of production... Finally, the supply price of one or more of the factors of production may rise, because the "other advantages" of a small firm are greater than those of a large firm.

Economists have uncovered the conditions necessary if Adam Smith’s results are to be achieved and where, in the real world, such conditions do not appear to be found, they have proposed changes which are designed to bring them about. It is what one finds in the textbooks. Harold Demsetz has said rightly that what this theory analyses is a system of extreme decentralisation. It has been a great intellectual achievement and it throws light on many aspects of the economic system. But it has not been by any means all gain.

The question always is, will it pay to bring an extra exchange transaction under the organizing authority? At the margin, the costs of organizing within the firm will be equal either to the costs of organizing in another firm or to the costs involved in leaving the transaction to be “organised” by the price mechanism. Business men will be constantly experimenting, controlling more or less, and in this way equilibrium will be maintained. This gives the position of equilibrium for static analysis.