Economic theory has suffered in the past from a failure to state clearly its assumptions. Economists in building up a theory have often omitted to examine the foundations on which it was erected. This examination is, however, essential not only to prevent the misunderstanding and needless controversy which arise from a lack of knowledge of the assumptions on which a theory is based, but also because of the extreme importance for economics of good judgement in choosing between rival sets of assumptions. For instance, it is suggested that the use of the word “firm” in economics may be different from the use of the term by the “plain man.”¹ Since there is apparently a trend in economic theory towards starting analysis with the individual firm and not with the industry,² it is all the more necessary not only that a clear definition of the word “firm” should be given but that its difference from a firm in the “real world,” if it exists, should be made clear. Mrs. Robinson has said that “the two questions to be asked of a set of assumptions in economics are: Are they tractable? and: Do they correspond with the real world?”³ Though, as Mrs. Robinson points out, “more often one set will be manageable and the other realistic,” yet there may well be branches of theory where assumptions may be both manageable and realistic. It is hoped to show in the following paper that a definition of a firm may be obtained which is not only realistic in that it corresponds to what is meant by a firm in the real world, but is tractable by two of the most powerful instruments of economic analysis developed by Marshall, the idea of the margin and that of substitution, together giving the idea of substitution at the margin.