The first imperfection: lighter sins that cling to good works
Thesis XIII. Their imperfection is of two kinds. First, lighter sins often spring up alongside good works and in some measure stain them by contact. They are like dust sprinkled over the works, dimming their brightness considerably.
So the mind often wanders during prayer. Stray thoughts frequently creep in while we listen to God's word. When we give alms, stirrings of distrust arise along with a certain itch for empty glory. In the same way, when we have to suffer for Christ, some wish for glory among people mixes itself with zeal for God's glory.
These are like swellings growing on good works of this kind, which wear away and reduce their worth.
Latin
XIII. Quippe duplex in iis occurrit imperfectio. Nam primo bonis operibus frequenter adnascuntur peccata quaedam leviora, quae illa suo contagio aliquo modo commaculant, & sunt veluti pulvis quidam iis aspersus, quo nitor eorum non parum obscuratur. Sic inter precandum saepe animus distrahitur. In audiendo Dei verbo non raro alienae cogitationes irrepunt. In largienda eleemosyna oriuntur motus quidam diffidentiae, & nonnulla inanis gloriae titillatio. Et similiter quum pro Christo patiendum est, divinae gloriae zelo immiscet se aliquis gloriae apud homines affectus. Quae sunt veluti strumae quaedam bonis ejusmodi operibus adnatae, quibus pretium eorum deteritur & imminuitur.