they saw with their eyes, touched with their hands, and to which
they erroneously attributed the faculty of perception, they were
prevented from distinctly apprehending the nature of the mind.

XIII. In what sense the knowledge of other things depends upon the
knowledge of God.

But when the mind, which thus knows itself but is still in doubt as
to all other things, looks around on all sides, with a view to the
farther extension of its knowledge, it first of all discovers within
itself the ideas of many things; and while it simply contemplates
them, and neither affirms nor denies that there is anything beyond
itself corresponding to them, it is in no danger of erring. The mind
also discovers certain common notions out of which it frames various
demonstrations that carry conviction to such a degree as to render
doubt of their truth impossible, so long as we give attention to
them. For example, the mind has within itself ideas of numbers and
figures, and it has likewise among its common notions the principle
THAT IF EQUALS BE ADDED TO EQUALS THE WHOLES WILL BE EQUAL and the
like; from which it is easy to demonstrate that the three angles of
a triangle are equal to two right angles, etc. Now, so long as we
attend to the premises from which this conclusion and others similar
to it were deduced, we feel assured of their truth; but, as the mind
cannot always think of these with attention, when it has the
remembrance of a conclusion without recollecting the order of its
deduction, and is uncertain whether the author of its being has
created it of a nature that is liable to be deceived, even in what
appears most evident, it perceives that there is just ground to
distrust the truth of such conclusions, and that it cannot possess
any certain knowledge until it has discovered its author.