Economics has as its purpose firstly to acquire knowledge for its own sake, and secondly to throw light on practical issues. But though we are bound, before entering on any study, to consider carefully what are its uses, we should not plan out our work with direct reference to them. For by doing so we are tempted to break off each line of thought as soon as it ceases to have an immediate bearing on that particular aim which we have in view at the time: the direct pursuit of practical aims leads us to group together bits of all sorts of knowledge, which have no connection with one another except for the immediate purposes of the moment; and which through but little light on one another. Our mental energy is spent in going from one to another; nothing is thoroughly thought through; no real progress is made.
Alfred Marshall, Principles of Economics, MacMillan, New York, 1961.