It has generally been recognized that the best expression of Kant's fundamental ethical principle is to be found in his third rule--Act as a member of a kingdom of ends: that is to say Act in such a way as to treat thyself and every other human being as of equal intrinsic value; behave as a member of a society in which each regards the good of each other as of equal value with its own, and is so treated by the rest, in which each is both end and means, in which each realizes his own good in promoting that of others. That such an ideal of human Society must, as far as it goes, be approved by the moral consciousness, follows from what has been already said: but, considered as a guide to the details of conduct, it suffers from the same fatal ambiguity as the preceding formulae. There is no sufficient definition or explanation of this good of others which we are to promote. We still have got nothing but a form without any content. If we fill up the deficiency from other parts of Kant's system, and interpret each man's end as goodness + happiness, that (as has been explained) gives us an intelligible, but a rough and inadequate, criterion of Morality: and on that interpretation, which in many passages would appear to be Kant's own^[110], we must cast to the winds the whole of his elaborate attempt to get at the details of conduct without appeal to experience or calculation of consequences, and to exhibit that good will as actuated by the mere form of a universal law without any regard to the content or matter of it.(Bk. 1 Ch. 5 § 7 ¶ 1)

In truth there run through the whole of Kant's ethical teaching two inconsistent and irreconcilable lines of thought--one of which is the basis (though only the basis) of all sound ethical theory; while the other has proved the fruitful parent of every extravagance, superstition, and absurdity by which the scientific study of Ethics has been, and still is, impeded. Every formula of Kant's may be interpreted, and at times appears to be interpreted by himself, in each of these opposite ways. Duty is a categorical imperative. That may mean there is a right course of action which is intrinsically right and reasonable for every man whether he likes it or not, and that is simply an analysis of what duty means to any one to whom it means anything at all. Or it may mean there are certain acts which we recognize as being right to do without thinking of the ends (social or otherwise) which they will tend to realize, than which no better definition could be given of the irrational in conduct. Duty for duty's sake may mean that we should pursue the good or intrinsically valuable end just because it is good, or it may mean that we should act without reference to an end at all. Act on a principle fit for a law universal may mean Pursue the ends which Reason pronounces to be intrinsically valuable for others no less than for thyself, or it may mean Make the avoidance of internal inconsistency the criterion of thy conduct. Treat humanity as an end and never merely as means may mean Regard the true Well-being of every man as possessing an intrinsic worth, or it may mean Regard it as beneath thy dignity to be of use to the society in which thou livest, and indulge in phantastic scruples about things which do no real harm to thyself or anybody else. The kingdom of ends represents simply a combination of the two last maxims, and is liable to the same charge of ambiguity; though of all the formulae employed by Kant it is the one which lends itself most readily to the more rational interpretation.(Bk. 1 Ch. 5 § 7 ¶ 2)