According to Alexis de Tocqueville, in the state of equality of conditions liberty is under serious threat. The passion for equality, which is the first and most intense passion of a democratic people, may prompt them to exchange liberty for despotism, in so far as they are willing to surrender themselves entirely to the search of material goods and self-interest, and to resign their power of judgment for the sake of the opinion of the majority, which is the real source of authority in such a social state. Taking this fragile balance between liberty and equality into account, this paper aims the consideration of American customs which allows Tocqueville to postulate a social theory (not a description of this society), the doctrine of interest properly understood, which may link analytically virtue and interest and instruct democratic societies.