affirmative precepts; negative precepts
Affirmative precepts enjoin acts to be performed, such as, "Remember thou keep holy the Sabbath Day."
Negative precepts forbid the placing of acts, as, "Thou shalt not commit adultery."
Still, not the phraseology but the concept must determine the affirmative or negative nature of a precept.
Affirmative precepts contain the positive will of the legislator, whereas negative precepts prohibit acts in themselves intrinsically wrong irrespective of positive legislation.
Despite this clear cut distinction it is difficult in some isolated cases to determine whether a precept is affirmative or negative.
"Thou shalt not steal" is a case in point.
Precepts, both affirmative and negative, urge at all times, but with this difference: negative precepts urge always and at every instant, since always and at every instant the prohibited act is to be omitted; but affirmative precepts urge always but not at every instant, since the enjoined act is always enjoined but not to be performed at every instant.
Affirmative precepts sometimes yield to excusing causes, but not even fear of death will excuse from the negative precepts of the Divine Law.
New Catholic Dictionary