Reason, in its speculative employment, conducted us through the field of experience, and since it could not find complete satisfaction there, from thence to speculative ideas, which, however, in the end brought us back to experience. In so doing the ideas fulfilled their purpose, but in a manner which, though useful, is not in accordance with our expectation. One other line of enquiry still remains open to us: namely, whether pure reason may not also be met with in the practical sphere, and whether it may not there conduct us to ideas which reach to those highest ends of pure reason that we have just stated, and whether, therefore, reason may not be able to supply to us from the standpoint of its practical interest what it altogether refuses to supply in respect of its speculative interest.

All the interests of my reason, speculative as well as practical, combine in the three following questions:

1. What can I know?
2. What ought I to do?
3. What may I hope?

The first question is merely speculative. We have, as I flatter myself, exhausted all the possible answers to it, and at last have found the answer with which reason must perforce content itself, and with which, so long as it takes no account of the practical, it has also good cause to be satisfied. But from the two great ends to which the whole endeavour of pure reason was really directed, we have remained just as far removed as if through love of ease we had declined this labour of enquiry at the very outset. So far, then, as knowledge is concerned, this much, at least, is certain and definitively established, that in respect of these two latter problems, knowledge is unattainable by us.

The second question is purely practical. As such it can indeed come within the scope of pure reason, but even so is not transcendental but moral, and cannot, therefore, in and by itself, form a proper subject for treatment in this Critique. The third question -- If I do what I ought to do, what may I then hope? -- is at once practical and theoretical, in such fashion that the practical serves only as a clue that leads us to the answer to the theoretical question, and when this is followed out, to the speculative question. For all hoping is directed to happiness, and stands in the same relation to the practical and the law of morality as knowing and the law of nature to the theoretical knowledge of things. The former arrives finally at the conclusion that something is (which determines the ultimate possible end) because something ought to happen; the latter, that something is (which operates as the supreme cause) because something happens.

Happiness is the satisfaction of all our desires, extensively, in respect of their manifoldness, intensively, in respect of their degree, and protensively, in respect of their duration. The practical law, derived from the motive of happiness, I term pragmatic (rule of prudence), and that law, if there is such a law, which has no other motive than worthiness of being happy, I term moral (law of morality). The former advises us what we have to do if we wish to achieve happiness; the latter dictates to us how we must behave in order to deserve happiness. The former is based on empirical principles; for only by means of experience can I know what desires there are which call for satisfaction; or what those natural causes are which are capable of satisfying them. The latter takes no account of desires, and the natural means of satisfying them, and considers only the freedom of a rational being in general, and the necessary conditions under which alone this freedom can harmonise with a distribution of happiness that is made in accordance with principles. This latter law can therefore be based on mere ideas of pure reason, and known a priori.