For as in bodily diseases there are many of which the affected are not sensible, but they rather put faith in the opinion of their physicians, than trust their own insensibility; so also in the diseases of the soul, though a man is not conscious of sin in himself, yet ought he to trust to those who are able to have more knowledge of their own sins.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: in Reg. Brev. ad int. 301.