It is the business of economics, as of almost every other science, to collect facts, to arrange and interpret them, and to draw inferences from them. “Observation and description, definition and classification are the preparatory activities. But what we desire to reach thereby is a knowledge of the interdependence of economic phenomena... Induction and deduction are both needed for scientific thought as the left and right foot are both needed for walking.”1 The methods required for this twofold work are not peculiar to economics; they are the common property of all sciences. All the devices for the discovery of the relations between cause and effect, which are described in treatises on scientific method, have to be used in their turn by the economist: there is not any one method of investigation which can properly be called the method of economics; but every method must be made serviceable in its proper place, either singly or in combination with others. And as the number of combinations that can be made on the chess-board is so great that probably no two games exactly alike were ever played; so no two games which the student plays with nature to wrest from her her hidden truths, which were worth playing at all, ever made use of quite the same methods in quite the same way.