(omne agens agit sibi simile).— The metaphysical principle according to which all effects have some formal likeness to their cause.  This principle is sometimes expressed thus: every cause likens its effects to itself.  Such likeness can be univocal when the form of the effect is the same as the cause (e.g., when an oak begets an oak) or equivocal-analogical (e.g., the world in relation God, artistic works in relation to man, etc.).