Quote by Immanuel Kant, Critique of Pure

Suppose some one asserts of his lustful appetite that, when the desired object and the opportunity are present, it is quite irresistible. Ask him if a gallows were erected before the house where he finds this opportunity, in order that he should be hanged thereon immediately after the gratification of his lust, whether he could not then control his passion; we need not be long in doubt what he would reply. Ask him, however, if his sovereign ordered him, on the pain of the same immediate execution, to bear false witness against an honourable man, whom the prince might wish to destroy under a plausible pretext, would he consider it possible in that case to overcome his love of life, however great it may be. He would perhaps not venture to affirm whether he would do so or not, but he would unhesitatingly admit that it is possible to do so. He judges, therefore, that he can do a certain thing because he is conscious that he ought, and he recognizes that he is free--a fact which but for the moral law he would never have known.