Thomas makes a reasonable observation about the relative certainty of articles of faith and deductions from reason:
Certitudo potest considerari dupliciter. Uno modo, ex causa certitudinis, et sic dicitur esse certius illud quod habet certiorem causam. Et hoc modo fides est certior tribus praedictis, quia fides innititur veritati divinae, tria autem praedicta innituntur rationi humanae. Alio modo potest considerari certitudo ex parte subiecti, et sic dicitur esse certius quod plenius consequitur intellectus hominis. Et per hunc modum, quia ea quae sunt fidei sunt supra intellectum hominis, non autem ea quae subsunt tribus praedictis, ideo ex hac parte fides est minus certa.
We can describe certainty as originating in the “cause” of our knowledge — this is faith’s objective component. So, if it is in fact the Word of God that we encounter in the articles of faith, then there is nothing left to be argued. However, ascertaining that this is so — that God is in fact speaking to us — is rather less certain than following the train of our own thought. This is faith’s subjective component.
I wonder if Thomas’s timidity on faith’s subjective component might have something to do with an incomplete pneumatology: after all, it properly is God the Holy Spirit, operating in and through us, who is himself the subjective ground of our faith, every bit as much as it is God the Father, speaking through the Son, his Word, who is faith’s objective ground.