Having determined the metaphysical point of view, the next question of vital importance is that of its _principle_. And we may cut matters short here by saying at once that the principle we are seeking is that of _sufficient reason_, and we may say that a reason will be sufficient when it adequately expresses the world-view or concept under which an investigation is being prosecuted. Let us suppose that this world-view is that of simple mathematics, the principle of sufficient reason here will be that of _quantitative equivalence_ of parts; or, from the standpoint of the whole, that of _infinite divisibility_. Whereas, if we take the world of the ultra-mathematical science, which is determined by the notion of _phenomena depending on underlying ground_, we will find that the sufficient reason in this sphere takes the form of _adequate cause or condition_. The determining condition or causes of any physical phenomenon supply, from that point of view, the _ratio sufficiens_ of its existence. We have seen that the sufficiency of a reason in the above cases has been determined in view of that notion which defines the kind of world the investigation is dealing with. Let us apply this insight to the problem of the principle of metaphysics, and we will soon conclude that no reason can be metaphysically sufficient that does not satisfy the requirements of a world conceived under the notion of _inception_ and _realization_; or, more specifically, _idea_ and _reality_. In short, the _reason_ of metaphysics will refuse to regard its world as a mechanism that is devoid of thought and intention; that lacks, in short, the motives of internal determination and movement, and will in all cases insist that an explanation or interpretation can be metaphysically adequate only when its ultimate reference is to an idea that is in the process of _purposive_ fulfillment. Such an explanation we call _teleological_ or _rational_, rather than merely mechanical, and such a principle is alone adequate to embody the _ratio sufficiens_ of metaphysics.