 A principle of induction is deduced from CP: it is necessary that similar causes have similar effects (hereafter CPI). Suppose two individually different causes are alike. If their effects were different, they would not have a cause; this is impossible according to CP. Often the same point is put this way: a difference must have a cause. If the qualities of an object are different at one time than another, the difference must have a cause; for the same reason, if individually different objects differ in respect of the presence or absence of any quality, the difference must have a cause. 