Alfred Marshall...on the tendencies of human action

The laws of economics are to be compared with the laws of the tides, rather than with the simple and exact law of gravitation. For the actions of men are so various and uncertain, that the best statement of tendencies , which we can make in a science of human conduct, must needs be inexact and faulty. This might be urged as a reason against making any statements at all on the subject. But that would be almost to abandon life. Life is human conduct, and the thoughts and emotions that grow up around it. By the fundemental impulses of our nature, we all, high and low, learned and unlearned, are in our several degrees constantly striving to understand the courses of human action, and to shape them for our purposes, whether selfish or unselfish, whether noble or ignoble. And since we must form to ourselves some notions of the tendencies of human action, our choice is between forming those notions carelessly and forming them carefully. The harder the task, the greater the need for steady, patient inquiry; for turning to account the experience that has been reaped by the more advanced physical sciences; and for framing as best we can well thought-out estimates, of provisional laws, of the tendencies of human action.

-Principles of Economics, 1892