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

In my view, what is wanted in industrial organization is a direct approach to the problem. This would concentrate on what activities firms undertake, and would endeavor to discover the characteristics of the groupings of activities within firms. Which activities tend to be associated and which do not? The answer may well differ for different kinds of firm.

Markets are institutions that exist to facilitate exchange, that is, they exist in order to reduce the cost of carrying out exchange transactions. In an economic theory which assumes that transaction costs are nonexistent. markets have no function to perform, and it seems perfectly reasonable to develop the theory of exchange by an elaborate analysis of individuals exchanging nuts for apples on the edge of the forest or some similar fanciful example. This analysis certainly shows why there is a gain from trade, but it fails to deal with the factors which determine how much trade there is or what goods are traded.

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.