Kant's Critique of Pure Reason
TRANSCENDENTAL LOGIC, SECOND DIVISION: TRANSCENDENTAL
DIALECTIC
BOOK II: THE DIALECTICAL INFERENCES OF PURE REASON
Chapter II: THE ANTINOMY OF PURE REASON
Section 8: THE REGULATIVE PRINCIPLE OF PURE REASON IN ITS APPLICATION TO THE COSMOLOGICAL IDEAS (p. 449)
The principle of reason is properly only a regulative rule.
No maximum of the series of conditions in a sensible world is given through the cosmological principle of totality, but can only be set as a task.
It is not a principle of the understanding because it is not a principle of empirical knowledge.
It is not a constitutive principle of reason which would allow us to extend our concept of the sensible world beyond experience.
It does not tell us what the object is, but only how the empirical
regress is to be carried out.
To determine how the synthesis is to be carried out when the synthesis is never complete:
Progressus in indefinitum: you produce the series as far as you please.
Progressus in infinitum: you must never cease producing
it.
When the whole is empirically given, it is possible to proceed back in the series of its inner conditions in infinitum.
The division of a body proceeds in infinitum because further members of the series are given empirically.
We necessarily find further members.
When the whole is not given (i.e. only a member is given), but has to be found through empirical regress, we can only say that the search for still higher conditions of the series is possible in infinitum.
The series of ancestors of any given man is not given in its absolute totality in any given possible experience, so no empirical limit is ever met, therefore you produce the series in indefinitum, i.e. search for additional members indefinitely far.
Since no experience is absolutely necessary, the necessity is that we enquire for them.