V. As a fourth source of evidence, I mentioned the moral sense, or that faculty of the mind, by which we have the original conceptions that there is a right and a wrong in conduct; and that some particular actions are right, and others wrong.
Without this last power of applying our conceptions to particular actions, and of determining concerning their moral qualities, our general and abstract notions of moral good and evil would be of no service to us in directing the conduct and affairs of human life.
The moral sense is a distinct and original power of the human mind. By this power, and by this power solely, we receive information and evidence of the first principles of right and wrong, of merit and demerit.
He, who would know the colour of any particular object, must consult his eye : in vain will he consult every other faculty upon the point.
In the same manner, he, who would learn the moral qualities of any particular action, must consult his moral sense : no other faculty of the mind can give him the necessary information.
The evidence given by our moral sense, like that given by our external senses, is the evidence of nature ; and, in both cases, we have the same grounds for relying on that evidence.
The truths given in evidence by the external senses are the first principles from which we reason concerning matter, and from which all our knowledge of the material world is drawn.
In the same manner, the truths given in evidence by our moral faculty are the first principles, from which we reason concerning moral subjects, and from which all our knowledge of morality is deduced.
The powers, which Nature has kindly bestowed upon us, are the only channels, through which the evidence of truth and knowledge can flow in upon our minds.
Virtuous demeanour is the duty, and should be the aim, of every man : the knowledge and evidence of moral truth is, therefore, placed within the reach of all.
Of right and wrong there are many different degrees; and there are also many different kinds. By the moral faculty we distinguish those kinds and degrees. By the same faculty we compare the different kinds together, and discover numerous moral relations between them.
Our knowledge of moral philosophy, of natural jurisprudence, of the law of nations, must ultimately depend, for its first principles, on the evidence and information of the moral sense.
This power furnishes to us the first principles of our most important knowledge. In dignity, it is far superiour to every other power of the human mind.
http://books.google.com/books?printsec=frontcover&id=82ZGAAAAYAAJ&output=text&pg=PA81