The old belief in the certainty of conscience now begins to reappear. It was Clarke (1675—1729) who attempted to defend it by the use of a mathematical analogy. We are all familiar with the axiom (or truism) that things which are equal to the same thing are equal to each other. If now we keep the general form, but alter the substance of this axiom, so that it reads: “Whatever I judge reasonable or unreasonable for another to do for me, that by the same judgment I declare reasonable or unreasonable that I in the like case should do for him,” we have Clarke’s Law of Equity. To an arm-chair philosopher such a statement sounds fairly cogent, but its empirical value is not thereby guaranteed. As every student of psychology knows, individual differences, individual preferences, and the capacity of individuals by this method of give-and-take are completely ignored in the formulation of this law. Besides, Clarke’s conception of conscience may be said to contain a hint of ethical solipsism, or the theory that oneself is the only accurate authority on social justice. We need not pause longer over the insuperable difficulties of such a point of view. It is, however, in the writings of Kant and Reid that the conscience-theory attains its modern, sentimental form. In his “Metaphysic of Morality” Kant (1724-1804) makes the following straddling assertion: “Nothing in the whole world, or even outside of the world [sic!] can possibly be regarded as good without limitation except a _good will_.” But this is not the full extent to which this philosopher presses his claims. For he continues, “A man’s will is good, not because the consequences which flow from it are good, nor because it is capable of attaining the ends which it seeks, but it is _good in itself_, or because it wills the good. Its intrinsic value is in no way increased by success or lessened by failure.” From all of which Kant concludes that “As the will must not be moved to act from any desire for the results expected to follow from obedience to a certain law, the only principle of the will which remains is that of the conformity of actions to universal law. In all cases I must act in such a way that I can at the same time will that my maxim should become a universal law.” This last statement is the famous categorical imperative.