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"
http://www.commens.org/dictionary/entry/quote-logic-chapter-i-thinking-cerebration