For the existence or non-existence of everything there must be a reason or cause. For example, if a triangle exists, there must be a reason or cause why it exists; and if it does not exist, there must be a reason or cause which hinders its existence or which negates it. But this reason or cause must either be contained in the nature of the thing or lie outside it. For example, the nature of the thing itself shows the reason why a square circle does not exist, the reason being that a square circle involves a contradiction. And the reason, on the other hand, why substance exists follows from its nature alone, which involves existence. But the reason why a circle or triangle exists or does not exist is not drawn from their nature, but from the order of corporeal nature generally; for from that it must follow either that a triangle necessarily exists, or that it is impossible for it to exist. But this is self-evident. Therefore it follows that if there be no cause nor reason which hinders a thing from existing, it exists necessarily. If therefore there be no reason nor cause which hinders God from existing, or which negates His existence, we must conclude absolutely that He necessarily exists. But if there be such a reason or cause, it must be either in the nature itself of God or must lie outside it, that is to say, in another substance of another nature. For if the reason lay in a substance of the same nature, the existence of God would be by this very fact admitted. But substance possessing another nature could have nothing in common with God, and therefore could not give Him existence nor negate it. Since, therefore, the reason or cause which could negate the divine existence cannot be outside the divine nature, it will necessarily, supposing that the divine nature does not exist, be in His nature itself, which would therefore involve a contradiction. But to affirm this of the Being absolutely infinite and consummately perfect is absurd. Therefore neither in God nor outside God is there any cause or reason which can negate His existence, and therefore God necessarily exists.... _The Corporeality of God_