We may now attempt to construct the demand curve for labour of a single unit of control, using the term demand curve in the illogical but convenient sense of the curve showing the amount of labour that would be employed at any given wage if the supply of labour to the unit of control were perfectly elastic at that wage. The unit of control is assumed to consist of a single unit of entrepreneurship, that is to say, it is a single firm. It may form part of an industry in which competition is perfect, or is imperfect, or it may be an isolated monopoly. The cost of entrepreneurship is assumed to be independent of output, and consequently of the number of men employed. With each number of men, as we have seen, there is a certain amount of capital which will be employed, such that its marginal productivity to the firm is equal to its marginal cost to the firm. Capital must now be taken to stand for all factors other than labour and entrepreneurship. The case of a firm which is part of a perfectly competitive industry is merely a particular example of a unit of control, but it will be easier, for our present purpose, to treat it separately before giving the general case.