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THE CATEGORICAL IMPERATIVE. 383 knowledge, yet not in conflict with it. The categories and the Ideas, moreover, yield problematical concepts of objects which are not given to us in intuition, but which may exist outside of space and time: things in themselves cannot be known, it is true, but they can be thought, a fact of impor- tance in case we should be assured of their existence in some other way than by sensuous intuition. The determination of the limits of speculative reason is finished. All knowing and all demonstration is limited to phenomena or possible experience. But the boundary of that which can be experienced is not the boundary of that which is, still less of that which ought to be ; the boundary of theoretical reason is not the boundary of practical reason. We ought to act morally ; in order to be able to do this we must ascribe to ourselves the power to initiate a series of events ; and, in general, we are warranted in assuming everything the non-assumption of which makes moral action impossible. If we were merely theo- retical, merely experiential beings, we should lack all occasion to suppose a second, intelligible world behind and above the world of phenomena ; but we are volitional and active beings under laws of reason, and though we are unable to know things in themselves, yet we may and must postulate them — our freedom, God, and immortality. For not only that which is a condition of experience is true and necessary, but that, also, which is a condition of morality. The discovery of the laws and conditions of morality is the mission of practical philosophy. ^ 2. Theory of Ethics. The investigation now turns from the laws of nature, which express a "must," to the laws of will, in which an " ought " is expressed, and by which certain actions are not compelled, but prescribed. (If we were merely rational, and not at the same time sensuous beings, the moral law would determine the will in the form of a natural law; since, however, the constant possibility of deviation is given in the sensibility, or, rather, the moral standpoint can only be attained by conquering the sensuous impulses, therefore the moral law speaks to us in the form of an " ought," of