He who made us would have been a pitiful bungler if he had made the rules of our moral conduct a matter of science. For one man of science, there are thousands who are not. What would have become of them? Man was destined for society. His morality therefore was to be formed to this object. He was endowed with a sense of right and wrong merely relative to this. This sense is as much a part of his nature as the sense of hearing, seeing, feeling; it is the true foundation of morality, and not the truth, &c., as fanciful writers have imagined. The moral sense, or conscience, is as much a part of man as his leg or arm. It is given to all human beings in a stronger or weaker degree, as force of members is given them in a greater or less degree. It may be strengthened by exercise, as may any particular limb of the body. This sense is submitted indeed in some degree to the guidance of reason; but it is a small stock which is required for this: even a less one than what we call Common sense.

Thomas Jefferson (1743-1826) American political philosopher, polymath, statesman, US President (1801-09)

Letter to Peter Carr (10 Aug 1787)
    (Source)