obligation; moral duty
Obligation or moral duty implies the practical imperative necessity of doing freely what is morally good and avoiding what is morally evil.
It is of the very essence of law, natural, ecclesiastical, or civil, to which all must necessarily be subject.
The sense of moral obligation is universal, being an attribute of man's rational nature, which is so constituted that the will is physically necessitated to tend towards perfect goodness and perfect happiness while being physically free in its choice of imperfect earthly goods.
Moral obligation constrains man to adopt the necessary means of obtaining his ultimate end by pursuing the line of conduct marked out by the moral law with its final goal man's perfect happiness.
The ultimate basis of moral obligation is the reason and will of our creator, who in fashioning human nature, imposed on man its right ordering and perfection as a sacred duty with its ultimate sanction the eternal possession or loss of God.
New Catholic Dictionary