Faith principles are not self-evident like science, yet we believe them immediately by the Spirit
Faith's principles and the sciences' differ here: the sciences' principles are self-known and evident, and by their own light compel the intellect to assent. Faith's principles, though they have some moral evidence, are — strictly and properly — inevident, and do not by their own light win the assent faith requires.
Yet they agree in this: as we assent to natural principles immediately by the natural light of the intellect, so we believe faith's principles immediately by the Spirit's illumination and impulse. And as nothing stops natural principles from sometimes being proved a posteriori, so faith's principles can be proved — but only a posteriori, and only ad hominem.
Latin
V. In hoc quidem differunt principia fidei, et principia scientiarum, quod principia scientiarum per se nota sunt et evidentia, et sua luce cogunt intellectum ad assensum: principia vero fidei, quamvis moralem aliquam evidentiam habeant, tamen, logice et proprie loquendo, sunt inevidentia, neque sua luce assensum, qualem fides requirit, impetrant. Attamen in hoc conveniunt quod sicut principiis naturalibus assentimur immediate, per naturale lumen intellectus: ita quoque fidei principiis immediate credimus, per illuminationem et impulsum Spiritus sancti. Verum ut nihil obstat quominus aliquando principia naturalia probentur a posteriori: sic etiam fidei principia probari quidem possunt, sed a posteriori, et ad hominem tantum.