But we very soon perceive that the chain or series of prosyllogisms,
that is, of deduced cognitions on the side of the grounds or conditions
of a given cognition, in other words, the ascending series of
syllogisms must have a very different relation to the faculty of reason
from that of the descending series, that is, the progressive procedure
of reason on the side of the conditioned by means of episyllogisms.
For, as in the former case the cognition (conclusio) is given only as
conditioned, reason can attain to this cognition only under the
presupposition that all the members of the series on the side of the
conditions are given (totality in the series of premisses), because
only under this supposition is the judgement we may be considering
possible à priori; while on the side of the conditioned or the
inferences, only an incomplete and becoming, and not a presupposed or
given series, consequently only a potential progression, is cogitated.
Hence, when a cognition is contemplated as conditioned, reason is
compelled to consider the series of conditions in an ascending line as
completed and given in their totality. But if the very same condition
is considered at the same time as the condition of other cognitions,
which together constitute a series of inferences or consequences in a
descending line, reason may preserve a perfect indifference, as to how
far this progression may extend _a parte posteriori_, and whether the
totality of this series is possible, because it stands in no need of
such a series for the purpose of arriving at the conclusion before it,
inasmuch as this conclusion is sufficiently guaranteed and determined
on grounds a parte priori. It may be the case, that upon the side of
the conditions the series of premisses has a first or highest
condition, or it may not possess this, and so be a parte priori
unlimited; but it must, nevertheless, contain totality of conditions,
even admitting that we never could succeed in completely apprehending
it; and the whole series must be unconditionally true, if the
conditioned, which is considered as an inference resulting from it, is
to be held as true. This is a requirement of reason, which announces
its cognition as determined à priori and as necessary, either in
itself—and in this case it needs no grounds to rest upon—or, if it is
deduced, as a member of a series of grounds, which is itself
unconditionally true.
Section III. System of Transcendental Ideas