Immanuel Kant (1724-1804), the founder of modern critical philosophy, has been studied for two centuries, with a current dictionary defining Kantianism in part as follows: ``The content of knowledge comes a posteriori from sense perception, but . . . its form is determined by a priori categories of the mind.'' Perhaps Kant's best-known phrase is ``categorical imperative,'' referring to a doctrine that one should act according to the same principles one would want everyone to follow. This passage on the subject is from ``Fundamental Principles of the Metaphysic of Morals'' (1785) in T. K. Abbott's familiar translation from the German.
The three modes of presenting the principle of morality that have been adduced are at bottom only so many formulae of the very same law, and each of itself involves the other two. There is, however, a difference in them, but it is rather subjectively than objectively practical, intended namely to bring an idea of the reason nearer to intuition (by means of a certain analogy), and thereby nearer to feeling. All maxims, in fact, have --
1. A form, consisting in universality; and in this view the formula of the moral imperative is expressed thus, that the maxims must be so chosen as if they were to serve as universal laws of nature.
2. A matter, namely, an end, and here the formula says that the rational being, as it is an end by its own nature and therefore an end in itself, must in every maxim serve as the condition limiting all merely relative and arbitrary ends.
3. A complete characterisation of all maxims by means of that formula, namely, that all maxims ought by their own legislation to harmonise with a possible kingdom of ends as with a kingdom of nature. There is a progress here in the order of the categories of unity of the form of the will (its universality), plurality of the matter (the objects, i.e. the ends), and totality of the system of these. In forming our moral judgment of actions it is better to proceed always on the strict method, and start from the general formula of the categorical imperative: Act according to a maxim which can at the same time make itself a universal law. If, however, we wish to gain an entrance for the moral law, it is very useful to bring one and the same action under the three specified conceptions, and thereby as far as possible to bring it nearer to intuition.
We can now end where we started at the beginning, namely, with the conception of a will unconditionally good. That will is absolutely good which cannot be evil -- in other words, whose maxim, if made a universal law, could never contradict itself. This principle, then, is its supreme law: Act always on such a maxim as thou canst at the same time will to be a universal law; this is the sole condition under which a will can never contradict itself; and such an imperative is categorical.