For instance, one, two, three, being given, everyone can see that the fourth proportional is six; and this is much clearer, because we infer the fourth number from an intuitive grasping of the ratio, which the first bears to the second.
PROP.XLI.

Knowledge of the first kind is the only source of falsity, knowledge of the second and third kinds is necessarily true.
Proof .-- To knowledge of the first kind we have (in the foregoing note) assigned all those ideas, which are inadequate and confused; therefore this kind of knowledge is the only source of falsity (II.

xxxv.).

Furthermore, we assigned to the second and third kinds of knowledge those ideas which are adequate; therefore these kinds are necessarily true (II.
xxxiv.).

Q.E.D.
PROP.XLII.