The question then is this:  Is it a necessary law for all rational beings that they should always judge of their actions by maxims of which they can themselves will that they should serve as universal laws?  If it is so, then it must be connected (altogether a priori) with the very conception of the will of a rational being generally.  But in order to discover this connexion we must, however reluctantly, take a step into metaphysic, although into a domain of it which is distinct from speculative philosophy, namely, the metaphysic of morals.  In a practical philosophy, where it is not the reasons of what happens that we have to ascertain, but the laws of what ought to happen, even although it never does, i. e., objective practical laws, there it is not necessary to inquire into the reasons why anything pleases or displeases, how the pleasure of mere sensation differs from taste, and whether the latter is distinct from a general satisfaction of reason; on what the feeling of pleasure or pain rests, and how from it desires and inclinations arise, and from these again maxims by the co-operation of reason:  for all this belongs to an empirical psychology, which would constitute the second part of physics, if we regard physics as the philosophy of nature, so far as it is based on empirical laws.  But here we are concerned with objective practical laws, and consequently with the relation of the will to itself so far as it is determined by reason alone, in which case whatever has reference to anything empirical is necessarily excluded; since if reason of itself alone determines the conduct (and it is the possibility of this that we are now investigating), it must necessarily do so a priori.

The will is conceived as a faculty of determining oneself to action in accordance with the conception of certain laws.  And such a faculty can be found only in rational beings.  Now that which serves the will as the objective ground of its self-determination is the end, and if this is assigned by reason alone, it must hold for all rational beings.  On the other hand, that which merely contains the ground of possibility of the action of which the effect is the end, this is called the means.  The subjective ground of the desire is the spring, the objective ground of the volition is the motive; hence the distinction between subjective ends which rest on springs and objective ends which depend on motives valid for every rational being.  Practical principles are formal when they abstract from all subjective ends, they are material when they assume these, and therefore particular springs of action.  The ends which a rational being proposes to himself at pleasure as effects of his actions (material ends) are all only relative, for it is only their relation to the particular desires of the subject that gives them their worth, which therefore cannot furnish principles universal and necessary for all rational beings and for every volition, that is to say practical laws.  Hence all these relative ends can give rise only to hypothetical imperatives.