o need to believe what can be proved by natural reason.

Obj. 3: Further, all things knowable scientifically [*Science is
certain knowledge of a demonstrated conclusion through its
demonstration] would seem to come under one head: so that if some of
them are proposed to man as objects of faith, in like manner the
others should also be believed. But this is not true. Therefore it is
not necessary to believe those things which can be proved by natural
reason.

_On the contrary,_ It is necessary to believe that God is one and
incorporeal: which things philosophers prove by natural reason.

_I answer that,_ It is necessary for man to accept by faith not only
things which are above reason, but also those which can be known by
reason: and this for three motives. First, in order that man may
arrive more quickly at the knowledge of Divine truth. Because the
science to whose province it belongs to prove the existence of God,
is the last of all to offer itself to human research, since it
presupposes many other sciences: so that it would not by until late
in life that man would arrive at the knowledge of God. The second
reason is, in order that the knowledge of God may be more general.
For many are unable to make progress in the study of science, either
through dullness of mind, or through having a number of occupations,
and temporal needs, or even through laziness in learning, all of whom
would be altogether deprived of the knowledge of God, unless Divine
things were brought to their knowledge under the guise of faith. The
third reason is for the sake of certitude. For human reason is very
deficient in things concerning God. A sign of this is that
philosophers in their researches, by natural investigation, into
human affairs, have fallen into many errors, and have disagreed among
themselves. And consequently, in order that men might have knowledge
of God, free of doubt and uncertainty, it was necessary for Divine
matters to be delivered to them by way of faith, being told to them,
as it were, by God Himself Who cannot lie.

Reply Obj. 1: The researches of natural reason do not suffice mankind
for the knowledge of Divine matters, even of those that can be proved
by reason: and so it is not superfluous if these others be believed.

Reply Obj. 2: Science and faith cannot be in the same subject and
about the same object: but what is an object of science for one, can
be an object of faith for another, as stated above (Q. 1, A. 5)