Faith's first principles stand like the axioms of a science
In every human art and science there are first principles proper to that field. From them every conclusion belonging to the art is drawn and proved. The principles themselves are certain in that kind and cannot be demonstrated. They are, as it were, the rule on which the rest hangs — though they neither can nor should prove themselves.
Divine teaching is the same. When its theorems are examined and we ask for their foundation, we must come to principles beyond which, in that kind at least, we cannot go.
Those principles cannot be proved by other principles of faith, and no one should demand that they prove themselves — nothing proves itself. Yet they must be held as a sufficient and perfect rule of faith, because everything else that belongs to faith is proved by them.
Latin
XVI. Quemadmodum igitur in disciplinis omnibus humanis sunt principia quaedam, unicuique arti & scientiae propria, ex quibus deducuntur & probantur conclusiones & propositiones omnes, quae pertinent ad artem & scientiam cujus sunt principia, ipsa vero in eo genere per se certa sunt nec demonstrari queunt: adeoque dici possunt esse veluti regula unde pendent, & per quam probantur omnia, quae ad talem artem aut scientiam spectant, licet ipsa sese probare nec possint, nec debeant: sic quoque res se habet in divina & caelesti doctrina. Nam quum Theoremata, & propositiones ejus ad examen revocantur, & quaeritur quod sit eorum fundamentum, necessario deveniendum est ad principia quaedam, ultra quae in eo saltem genere, procedi non possit: Ac per consequens, quae non possint per alia fidei principia probari, & a quibus quoque exigi non debet, ut seipsa probent, quandoquidem seipsum nihil probare potest: Et quae tamen haberi debeant pro sufficiente & perfecta fidei regula, quoniam caetera omnia, quae ad fidem pertinent, per ipsa probantur atque demonstrantur.