The Express Moral Principles of which I have spoken, as the basis of Duties, are those which express, in an imperative form, the five Cardinal Virtues: namely, the Principle of Humanity, that Man is to be loved as Man : the Principle of Justice, that Each Man is to have his own: the Principle of Truth, that We must conform to the universal Understanding which the use of Language among men implies: the Principle of Purity, that the Lower Parts of our nature are to be governed by the Higher: and the Principle of Order, that We must obey positive Laws as the necessary conditions of Morality. I have, in a former Lecture, spoken of the degree and kind of the evidence of the first of these Express Principles; and the like remarks might be made upon the others. They commend themselves to our assent, in proportion as our moral nature is cultivated and educed: they become evident to us when we think and feel as really moral creatures. The perception of them may be obscured by the influence of the ferine part of our nature ;—by savage rudeness, passion, partiality: but in proportion as the ferine element is subdued, and the human element brought out in its proper force, these Principles are accepted. When man judges as man and for man, he is enabled to see their full meaning; and with their meaning, their truth.
William Whewell, Lectures on Systematic Morality, Lecture V.