Broadly speaking, Smith followed the views of his mentor, Francis Hutcheson of the University of Glasgow, who divided moral philosophy into four parts: Ethics and Virtue; Private rights and Natural liberty; Familial rights (called Economics); and State and Individual rights (called Politics).
Hutcheson had abandoned the psychological view of moral philosophy, claiming that motives were too fickle to be used as a basis for a philosophical system. Instead, he hypothesised a dedicated "sixth sense" to explain morality. This idea, to be taken up by David Hume (see Hume's A Treatise of Human Nature), claimed that man is pleased by utility.
Smith rejected his teacher's reliance on this special sense. Starting in about 1741, Smith set on the task of using Hume's experimental method (appealing to human experience) to replace the specific moral sense with a pluralistic approach to morality based on a multitude of psychological motives. The Theory of Moral Sentiments begins with the following assertion:
How selfish soever man may be supposed, there are evidently some principles in his nature, which interest him in the fortunes of others, and render their happiness necessary to him, though he derives nothing from it, except the pleasure of seeing it. Of this kind is pity or compassion, the emotion we feel for the misery of others, when we either see it, or are made to conceive it in a very lively manner. That we often derive sorrow from the sorrows of others, is a matter of fact too obvious to require any instances to prove it; for this sentiment, like all the other original passions of human nature, is by no means confined to the virtuous or the humane, though they perhaps may feel it with the most exquisite sensibility. The greatest ruffian, the most hardened violator of the laws of society, is not altogether without it.
Smith departed from the "moral sense" tradition of Shaftesbury, Hutcheson, and Hume, as the principle of sympathy takes the place of that organ. "Sympathy" was the term Smith used for the feeling of these moral sentiments. It was the feeling with the passions of others. It operated through a logic of mirroring, in which a spectator imaginatively reconstructed the experience of the person he watches:
As we have no immediate experience of what other men feel, we can form no idea of the manner in which they are affected, but by conceiving what we ourselves should feel in the like situation. Though our brother is on the rack, as long as we ourselves are at our ease, our senses will never inform us of what he suffers. They never did, and never can, carry us beyond our own person, and it is by the imagination only that we can form any conception of what are his sensations. Neither can that faculty help us to this any other way, than by representing to us what would be our own, if we were in his case. It is the impressions of our own senses only, not those of his, which our imaginations copy. By the imagination, we place ourselves in his situation.
However, Smith rejected the idea that Man was capable of forming moral judgements beyond a limited sphere of activity, again centered around his own self-interest:
The administration of the great system of the universe ... the care of the universal happiness of all rational and sensible beings, is the business of God and not of man. To man is allotted a much humbler department, but one much more suitable to the weakness of his powers, and to the narrowness of his comprehension: the care of his own happiness, of that of his family, his friends, his country.... But though we are ... endowed with a very strong desire of those ends, it has been entrusted to the slow and uncertain determinations of our reason to find out the proper means of bringing them about. Nature has directed us to the greater part of these by original and immediate instincts. Hunger, thirst, the passion which unites the two sexes, and the dread of pain, prompt us to apply those means for their own sakes, and without any consideration of their tendency to those beneficent ends which the great Director of nature intended to produce by them.
The rich only select from the heap what is most precious and agreeable. They consume little more than the poor, and in spite of their natural selfishness and rapacity, though they mean only their own conveniency, though the sole end which they propose from the labours of all the thousands whom they employ, be the gratification of their own vain and insatiable desires, they divide with the poor the produce of all their improvements. They are led by an invisible hand to make nearly the same distribution of the necessaries of life, which would have been made, had the earth been divided into equal portions among all its inhabitants, and thus without intending it, without knowing it, advance the interest of the society, and afford means to the multiplication of the species.
In a published lecture, Vernon L. Smith further argued that Theory of Moral Sentiments and Wealth of Nations together encompassed:
"one behavioral axiom, 'the propensity to truck, barter, and exchange one thing for another,' where the objects of trade I will interpret to include not only goods, but also gifts, assistance, and favors out of sympathy ... whether it is goods or favors that are exchanged, they bestow gains from trade that humans seek relentlessly in all social transactions. Thus, Adam Smith's single axiom, broadly interpreted ... is sufficient to characterize a major portion of the human social and cultural enterprise. It explains why human nature appears to be simultaneously self-regarding and other-regarding."