Thesis XVII. A defect of the first kind concerns the act's kind and essence, and robs it of some essential perfection. A defect of the second kind is one of degree and is accidental: it removes from the act only an accidental, individual perfection.
So an act that lacks something in the first way is bad, so bad that it cannot be called good without qualification. An act that lacks something only in the second way does not stop being truly and properly good, though its goodness is imperfect.
Latin
XVII. Defectus prioris generis specificus est & essentialis & actionem aliqua essentiali perfectione privat. Sed posterioris generis defectus gradualis est & accidentalis & ab actione tollit perfectionem tantum accidentalem & individualem. Adeoque actus ille cui aliquid priori modo deest, ita malus est ut simpliciter bonus dici non possit: si vero solum posteriori modo ipsi aliquid desit, non desinit esse vere & proprie bonus, licet imperfectam tantum bonitatem habeat.