A want or requirement of pure reason in its speculative use leads
only to a hypothesis; that of pure practical reason to a postulate;
for in the former case I ascend from the result as high as I please in
the series of causes, not in order to give objective reality to the
result (e.g., the causal connection of things and changes in the
world), but in order thoroughly to satisfy my inquiring reason in
respect of it. Thus I see before me order and design in nature, and
need not resort to speculation to assure myself of their reality,
but to explain them I have to presuppose a Deity as their cause; and
then since the inference from an effect to a definite cause is
always uncertain and doubtful, especially to a cause so precise and so
perfectly defined as we have to conceive in God, hence the highest
degree of certainty to which this pre-supposition can be brought is
that it is the most rational opinion for us men. * On the other hand,
a requirement of pure practical reason is based on a duty, that of
making something (the summum bonum) the object of my will so as to
promote it with all my powers; in which case I must suppose its
possibility and, consequently, also the conditions necessary
thereto, namely, God, freedom, and immortality; since I cannot prove
these by my speculative reason, although neither can I refute them.
This duty is founded on something that is indeed quite independent
of these suppositions and is of itself apodeictically certain, namely,
the moral law; and so far it needs no further support by theoretical
views as to the inner constitution of things, the secret final aim
of the order of the world, or a presiding ruler thereof, in order to
bind me in the most perfect manner to act in unconditional
conformity to the law. But the subjective effect of this law,
namely, the mental disposition conformed to it and made necessary by
it, to promote the practically possible summum bonum, this
pre-supposes at least that the latter is possible, for it would be
practically impossible to strive after the object of a conception
which at bottom was empty and had no object. Now the above-mentioned
postulates concern only the physical or metaphysical conditions of the
possibility of the summum bonum; in a word, those which lie in the
nature of things; not, however, for the sake of an arbitrary
speculative purpose, but of a practically necessary end of a pure
rational will, which in this case does not choose, but obeys an
inexorable command of reason, the foundation of which is objective, in
the constitution of things as they must be universally judged by
pure reason, and is not based on inclination; for we are in nowise
justified in assuming, on account of what we wish on merely subjective
grounds, that the means thereto are possible or that its object is
real. This, then, is an absolutely necessary requirement, and what
it pre-supposes is not merely justified as an allowable hypothesis,
but as a postulate in a practical point of view; and admitting that
the pure moral law inexorably binds every man as a command (not as a
rule of prudence), the righteous man may say: "I will that there be
a God, that my existence in this world be also an existence outside
the chain of physical causes and in a pure world of the understanding,
and lastly, that my duration be endless; I firmly abide by this, and
will not let this faith be taken from me; for in this instance alone
my interest, because I must not relax anything of it, inevitably
determines my judgement, without regarding sophistries, however unable
I may be to answer them or to oppose them with others more
plausible. *(2)