EXCUSE FROM DUTY
Fact or circumstance that allows a person not to fulfill a certain obligation. Duty is imposed by law, which by definition is reasonable and for the common good. The purpose is not to crush men and women with unreasonable burdens. Hence there are causes that can excuse from duty, because in these cases the duty really invades a person's rights. In general, legitimate excuses are reducible to practical impossibility or disproportionate hardship. But much depends on whether the natural or a positive law is in question.
A negative duty (prohibition) arising from the natural law admits of no excuse whatever. Such a duty concerns matters forbidden by the natural law as inherently wrong. Such are blasphemy, perjury, and murder. An affirmative duty (command) arising form the natural law admits of excuse, but there must be no violation of a prohibition imposed by the natural law. For example the omission of an act of worship must not be interpreted as a denial of God. Duties arising from human positive law, whether prohibitions or commands, admit of excuse because of impossibility or excessive hardships. But here, too, there can be no excuse from a prohibition binding by the natural law.