From the ‘Introduction’

The word ‘cause’ has been used in the preceding paragraphs to indicate proximate causes only. This means that the economic considerations upon which the relation tested is based must be directed towards finding, as far as possible ‘direct causal relationships’. The variables in the relation must be directly connected either in the minds of some persons (e.g., through the reaction of the consumer to a given income and price) or by some definition (e.g., value of sales equals volume times price). This is not always possible if the strictest sense of ‘direct’ is kept to. Investment activity may be linked up directly with profit expectations, and these are hardly measurable. The next step connecting profit expectations with actual profits and some other variables may then also be included, and investment activity may be ‘explained’ both by actual profits and by some other variables. The more, however, such combinations of successive steps can be avoided in the formulation of relations, the better. This combination may always be undertaken afterwards – in fact, it forms the very important next step in our work – but the more explicitly it is done, the better. By keeping to this principle, one obtains relations with what Professor Frisch calls the maximum degree of ‘autonomy’ – i.e., relations which are as little as possible affected by structural changes in departments of economic life other than the one they belong to. It is clearly the task of economic analysis to indicate the nature of those direct causal relationships.