Democratic, too, is the concept of human nature implicit in Smith's political economy. The much quoted phrase, "the propensity to truck, barter, and exchange one thing for another," appears early in the book in the section on the division of labour. Smith refused to speculate about the origin of that "certain propensity in human nature" — whether it is innate in human nature or a consequence of the "faculties of reason and speech". In either case, it is a basic trait "common to all men" — just as the "moral sentiments", in the earlier book, are common to all men. These are the modest attributes that make the labourer a fully moral being, capable and desirous of bettering himself, of exercising his interests, passions, and virtues, and of enjoying the liberty that is his right as a free individual and a responsible member of society. No enlightened despot, not even an enlightened philosopher or legislator, is required to activate these qualities or to regulate and harmonise them for the general good.

This common human nature has even more dramatic implications, for it means that all men share not only these modest virtues, but also the "natural talents" that make for a distinctive "genius and disposition".

The difference of natural talents in different men is, in reality, much less than we are aware of; and the very different genius which appears to distinguish men of different professions, when grown up to maturity, is not upon many occasions so much the cause as the effect of the division of labour. The difference between the most dissimilar characters, between a philosopher and a common street porter, for example, seems to arise not so much from nature, as from habit, custom, and education. . . By nature a philosopher is not in genius and disposition half so different from a street porter, as a mastiff is from a greyhound, or a greyhound from a spaniel, or this last from a shepherd's dog.

Even today this stands as a bold assertion of the priority of nurture over nature, an affirmation of the natural equality of all people — not, to be sure, political, economic, or social equality, but a basic equality of human nature. Hume had earlier made a similar observation: "How nearly equal all men are in their bodily force, and even in their mental power and faculties, till cultivated by education." This may be one of the defining differences between the British and French Enlightenments. One of the most eminent of the French philosophes, writing about the same time as Smith, rejected any notion of the natural equality of men. When Helvétius was bold enough to suggest that circumstance, education, and interest account for differences in "l'esprit," Diderot rebuked him: "He has not seen the insurmountable barrier that separates a man destined by nature for a given function, from a man who only brings to that function industry, interest and attention."

A philosopher "not so different from a street porter" — this does not have quite the ring of that "self-evident" truth, "All men are created equal." Yet it does seem providential that The Wealth of Nations should have been published only months before the American Declaration of Independence. For its time and place, without the provocation of revolution or radical dissent, Smith's statement is memorable, all the more because it comes from one of Britain's most eminent philosophers — a political economist who was truly a moral philosopher.