Thomas Aquinas

“ Now every lack of a due act is sinful: wherefore it is evident that negligence is a sin, and that it must needs have the character of a special sin according as solicitude is the act of a special virtue. For certain sins are special through being about a special matter, as lust is about sexual matters, while some vices are special on account of their having a special kind of act which extends to all kinds of matter, and such are all vices affecting an act of reason, since every act of reason extends to any kind of moral matter. ”

“ Properly speaking the matter of negligence is a good that one ought to do, not that it is a good when it is done negligently, but because on account of negligence it incurs a lack of goodness, whether a due act be entirely omitted through lack of solicitude, or some due circumstance be omitted. ”

“ When negligence consists in the omission of that which is necessary for salvation, it is drawn to the other more manifest genus of sin. Because those sins that consist of inward actions, are more hidden, wherefore no special sacrifices were prescribed for them in the Law, since the offering of sacrifices was a kind of public confession of sin, whereas hidden sins should not be confessed in public. ”