Law of Habit
According to the universal law of habit, all vital processes whatever tend to become easier on repetition. (J. J. Murphy. Habit and Intelligence. 1869. Chap. XV. Vol. I. p. 169.). The nervous system is particularly susceptible to habit, and along whatever path a nervous discharge has once taken place, along that path a new discharge is the more likely to take place. It is an essential characteristic of this law that it is not absolute.
Joseph John Murphy's original formulation, cited in W 4: "The definition of habit, and its primary law, is that all vital actions tend to repeat themselves; or, if they are not such as can repeat themselves, they tend to become easier on repetition"
I believe the law of habit to be purely psychical. But then I suppose matter is merely mind deadened by the development of habit. While every physical process can be reversed without violation of the law of mechanics, the law of habit forbids such a reversal.