"Man is an animal," for animal is contained in the essence of man. If,
therefore the essence of the predicate and subject be known to all,
the proposition will be self-evident to all; as is clear with regard
to the first principles of demonstration, the terms of which are
common things that no one is ignorant of, such as being and non-being,
whole and part, and such like. If, however, there are some to whom the
essence of the predicate and subject is unknown, the proposition will
be self-evident in itself, but not to those who do not know the
meaning of the predicate and subject of the proposition. Therefore, it
happens, as Boethius says (Hebdom., the title of which is: "Whether
all that is, is good"), "that there are some mental concepts
self-evident only to the learned, as that incorporeal substances are
not in space." Therefore I say that this proposition, "God exists," of
itself is self-evident, for the predicate is the same as the subject,
because God is His own existence as will be hereafter shown (Q. 3,
Art. 4). Now because we do not know the essence of God, the proposition
is not self-evident to us; but needs to be demonstrated by things that
are more known to us, though less known in their nature--namely, by
effects.

Reply Obj. 1: To know that God exists in a general and
confused way is implanted in us by nature, inasmuch as God is man's
beatitude. For man naturally desires happiness, and what is naturally
desired by man must be naturally known to him. This, however, is not
to know absolutely that God exists; just as to know that someone is
approaching is not the same as to know that Peter is approaching, even
though it is Peter who is approaching; for many there are who imagine
that man's perfect good which is happiness, consists in riches, and
others in pleasures, and others in something else.