Reason guides us in the fulfillment of our goals or ends. It prescribes "maxims" or principles on whose basis we act rationally. For the most part, these maxims are practical: "Wear nice clothing to an interview," "Don't cross the freeway on foot," "Be nice to your teacher." Each says what one ought to do, if one wishes to attain a given end. "Wear nice clothing to an interview, if you want to get the job." Kant calls such imperatives "hypothetical," and he declares that they have no moral worth.
What does have moral worth for Kant is actions whose imperative is "categorical," without an implicit 'if' clause attached to them. One should act according to a maxim whose application is universal. As Kant put it, you should be able to will that your maxim be a universal law, holding good for all persons at all places and all times. An example of such a maxim is "Do not deliberately deceive anyone." One refrains from deception not because lying often causes a lot of trouble, or because honest people are well-liked and respected, but because everyone ought to refrain from deception at all times.
Schopenhauer was sharply critical of the Kantian doctrine. In the first place, he held that underneath the rationalistic-sounding categorical imperative lay a theological ethics. "Thou shalt," and "Thou shalt not," really are the commands of God. "But from theological morals Kant had borrowed this imperative form of ethics tacitly and without examining it. The hypotheses of such morals and hence theology really underlie that form, and in fact as that whereby alone it has sense and meaning, they are inseparable from it; indeed they are implicitly contained in it" (On The Basis of Morality, Section 4).
Another criticism concerned the way in which Kant described a categorical imperative as one which could be willed to be a universal law. The basis of this willing could only be egoism, which according to Schopenhauer is the fundamental anti-moral incentive. For example, in discussing the imperative enjoining us not to deceive others, Kant stated, "I cannot wish for a general law to establish lying because no one would any longer believe me, or I should be paid in the same coin" (quoted by Schopenhauer from Kant's Groundwork of the Metaphysics of Morals).
Thus it is the potential consequences to myself which underlie the maxim not to lie, and this renders the imperative hypothetical after all. "For tacitly underlying it is the condition that the law to be laid down for my action, since I raise it to one that is universal, also becomes the law for my suffering, and on this condtion I, as the eventualiter passive part, certainly cannot will injustice and uncharitableness" ( On The Basis of Morality, Section 4).
The general objection which underlies Schopenhauer's specific criticisms of Kant is that it is futile to attempt to base morality on reason. The difference is really metaphysical. Recall that Kant distinguished between appearances and things in themselves. All changes in appearances are necessary, but insofar as things in themselves are not subject to the laws dictated by the human understanding, they might be free. Kant postulated that human beings, in themselves, are free. He did so because he maintained that freedom of action is an absolute prerequisite of morality. One is free insofar as one acts on the basis of reason alone, independently of any sensuous impulses. Human reason is autonomous insofar as it acts on imperatives that it lays down itself.
For Schopenhauer, the metaphysical picture was quite different. The thing in itself is identified with will, and will is free insofar as it is not subject to the necessity of causality. The acts of the will, on this view, are utterly groundless: the will obeys no law of reason, and indeed it obeys no law whatsoever. The underlying reality is entirely irrational. The function of reason is to discover, through the attainment of knowledge, the most effective ways of carrying out the impulses of the will. Thus the maxims of reason are all hypothetical.
But this does not mean that morality is impossible, only that it is not based on rational knowledge. Instead, it is based on the intuitive knowledge we have of ourselves as will. When one recognizes that will is free from the confines of space and time, and thus that individuality is illusory, one feels the unity of all things. And it is from this knowledge of unity that moral behavior springs.
There are two maxims of morality, one negative (also rational and masculine), one positive (also intuitive and feminine). The maxim of justice is: "Injure no one." That of loving-kindness is, "Help everyone as much as you can." Justice is based on the calculation of the consequences of actions, while loving-kindness springs from a spontaneous recognition of suffering. Both have their metaphysical basis in the feeling of compassion, resulting from the recognition of the oneness of all things.
Moral acts have a peculiar feature. They invert the usual relationship between will and intellect. In non-moral action, the intellect throught its knowledge merely facilitates the impulses of the will. But knowledge of the unity of all things turns us against these impulses. The will is betrayed by its very product, the knowing subject.
Moreover, there is literally nothing to be gained through the denial of the will. Insofar as will exhausts reality, its denial is a denial of reality as such. Schopenhauer credits this insight to mystics and saints through the ages. The Buddhists call this nothing "Nirvana," with the hint that it is a something, a paradise. But Schopenhauer faces it squarely and declares that the goal is literally nothing, the abolition of reality altogether.