A firm may be defined as an institution which buys things, transforms them in some way, and then sells them with the purpose of making a profit. The things a firm buys we shall call "inputs." The things it sells we shall call "outputs." The process whereby the things it buys are transformed into the things it sells we shall call the "process of production." In any process of buying to sell again a process of production is always involved...
Kenneth Boulding