So far, we have discussed the changes shown by the variables of a state-determined system, and have ignored the fact that all its changes occur on a background, or on a foundation, of constancies. Thus, a particular simple pendulum provides two variables which are known (S. 2/15) to be such that, if we are given a particular state of the system, we can predict correctly its ensuing behaviour; what has not been stated explicitly is that this is true only if the length of the string remains constant. The background, and these constancies, must now be considered.