Any one idea or, according to our psychology, any one wave that is produced in the mind-stuff, Chitta, must always give rise to many similar waves. This is the psychological idea of association, and causation is only an aspect of this grand pervasive principle of association. This pervasiveness of association is what is, in Sanskrit, called Vyâpti. In the external world the idea of law is the same as in the internal — the expectation that a particular phenomenon will be followed by another, and that the series will repeat itself. Really speaking, therefore, law does not exist in nature. Practically it is an error to say that gravitation exists in the earth, or that there is any law existing objectively anywhere in nature. Law is the method, the manner in which our mind grasps a series of phenomena; it is all in the mind. Certain phenomena, happening one after another or together, and followed by the conviction of the regularity of their recurrence — thus enabling our minds to grasp the method of the whole series — constitute what we call law.