Therefore God necessarily exists.
Another proof .-- Of everything whatsoever a cause or reason must be assigned, either for its existence, or for its non--existence--e.g.

if a triangle exist, a reason or cause must be granted for its existence; if, on the contrary, it does not exist, a cause must also be granted, which prevents it from existing, or annuls its existence.

This reason or cause must either be contained in the nature of the thing in question, or be external to it.

For instance, the reason for the non--existence of a square circle is indicated in its nature, namely, because it would involve a contradiction.

On the other hand, the existence of substance follows also solely from its nature, inasmuch as its nature involves existence.