In dispute we often prove what is prior by what is posterior
So not every proof is taken from what is simply better known and more certain.
In dispute we must adapt to those we are dealing with. Against people who deny principles — even things most certain in themselves — we use weaker proofs and arguments that rely on what is known later: proving what is prior in knowledge, simply speaking, through what is known later.
Against Gentiles who deny the Christian religion we often argue from the opinions of the wise, from philosophical axioms, and from the nations' consensus — yet none of that has, for us, as much certainty as the Christian religion itself.
Even first principles, than which nothing is absolutely better known or more certain, we illustrate by examples and inductions; but their certainty and authority do not depend on those illustrations.
Latin
XLIII. Itaque dicendum est non omnes probationes ab eo sumi quod est simpliciter notius & certius. Etenim inter disputandum cogimur nos attemperare illis, quibuscum nobis res est: Et contra eos qui principia negant, & ea quae sunt in se certissima, uti minus certis probationibus, & argumentis illis quae vocantur a posteriori, hoc est, quibus quod est cognitione prius, simpliciter loquendo, probatur per illud quod est cognitione posterius. Sic contra Ethnicos qui negant religionem Christianam, argumentamur saepe ex placitis sapientum, philosophicis axiomatibus, & consensu gentium. Et tamen omnia ista tantam respectu nostri certitudinem non habent, quam religio Christiana. Et ipsa prima principia, quibus absolute nihil est notius & certius, probamus exemplis & inductionibus, unde tamen non pendet principiorum illorum certitudo & authoritas.