Economics, like other sciences, has certain important laws which must be mastered as we progress in our study. It is desirable, then, at the outset to understand as accurately as possible the nature of an economic law. Scientific laws as a rule merely state tendencies. We learn from the study of physics that a metal rod shortens as its temperature falls. Suppose, however, that the two ends of such a rod be firmly secured. Obviously, then, a lowering of the temperature would not cause the rod to become shorter. We must not conclude, however, that the law is invalid, for it states only a tendency which would have become a reality had no opposing force been exerted on the rod. Similarly in economics, laws hold good only so long as they are not neutralized by opposing ones. For that reason it is best to qualify an economic law with the expression "tends," or "other things remaining equal." Thus, to illustrate, we should not say that the fall in price of a particular commodity will increase the demand for that commodity. Ordinarily such a statement would be correct, but not always. If at the same time the prices of all other commodities should fall as rapidly, there is no reason to believe that there would be any increase in the demand. It would be more correct, therefore, to say that as the price of a particular commodity falls, the demand tends to rise; or, as the price of a particular commodity falls, the demand, other things remaining equal, will rise.