have a right to demand a deduction of the principles employed in it；
and； if these principles have their origin in pure reason alone；
such a deduction is absolutely impossible。 And thus it is
unnecessary that we should trouble ourselves with the exposure and
confutation of every sophistical illusion； we may； at once； bring
all dialectic； which is inexhaustible in the production of
fallacies； before the bar of critical reason； which tests the
principles upon which all dialectical procedure is based。 The second
peculiarity of transcendental proof is that a transcendental
proposition cannot rest upon more than a single proof。 If I am drawing
conclusions； not from conceptions； but from intuition corresponding to
a conception； be it pure intuition； as in mathematics； or empirical；
as in natural science； the intuition which forms the basis of my
inferences presents me with materials for many synthetical
propositions； which I can connect in various modes； while； as it is
allowable to proceed from different points in the intention； I can
arrive by different paths at the same proposition。
But every transcendental proposition sets out from a conception； and
posits the synthetical condition of the possibility of an object
according to this conception。 There must； therefore； be but one ground
of proof； because it is the conception alone which determines the
object； and thus the proof cannot contain anything more than the
determination of the object according to the conception。 In our
Transcendental Analytic； for example； we inferred the principle： Every
event has a cause； from the only condition of the objective
possibility of our conception of an event。 This is that an event
cannot be determined in time； and consequently cannot form a part of
experience； unless it stands under this dynamical law。 This is the
only possible ground of proof； for our conception of an event
possesses objective validity； that is； is a true conception； only
because the law of causality determines an object to which it can
refer。 Other arguments in support of this principle have been
attempted… such as that from the contingent nature of a phenomenon；
but when this argument is considered； we can discover no criterion
of contingency； except the fact of an event… of something happening；
that is to say； the existence which is preceded by the non…existence
of an object； and thus we fall back on the very thing to be proved。 If
the proposition： 〃Every thinking being is simple；〃 is to be proved； we
keep to the conception of the ego； which is simple； and to which all
thought has a relation。 The same is the case with the transcendental
proof of the existence of a Deity； which is based solely upon the
harmony and reciprocal fitness of the conceptions of an ens
realissimum and a necessary being； and cannot be attempted in any
other manner。
This caution serves to simplify very much the criticism of all
propositions of reason。 When reason employs conceptions alone； only
one proof of its thesis is possible； if any。 When； therefore； the
dogmatist advances with ten arguments in favour of a proposition； we
may be sure that not one of them is conclusive。 For if he possessed
one which proved the proposition he brings forward to demonstration…
as must always be the case with the propositions of pure reason…
what need is there for any more？ His intention can only be similar
to that of the advocate who had different arguments for different
judges； this availing himself of the weakness of those who examine his
arguments； who； without going into any profound investigation； adopt
the view of the case which seems most probable at first sight and
decide according to it。
The third rule for the guidance of pure reason in the conduct of a
proof is that all transcendental proofs must never be apagogic or
indirect； but always ostensive or direct。 The direct or ostensive
proof not only establishes the truth of the proposition to be
proved； but exposes the grounds of its truth； the apagogic； on the
other hand； may assure us of the truth of the proposition； but it
cannot enable us to prehend the grounds of its possibility。 The
latter is； accordingly； rather an auxiliary to an argument； than a
strictly philosophical and rational mode of procedure。 In one respect；
however； they have an advantage over direct proofs； from the fact that
the mode of arguing by contradiction； which they employ； renders our
understanding of the question more clear； and approximates the proof
to the certainty of an intuitional demonstration。
The true reason why indirect proofs are employed in different
sciences is this。 When the grounds upon which we seek to base a
cognition are too various or too profound； we try whether or not we
may not discover the truth of our cognition from its consequences。 The
modus ponens of reasoning from the truth of its inferences to the
truth of a proposition would be admissible if all the inferences
that can be drawn from it are known to be true； for in this case there
can be only one possible ground for these inferences； and that is
the true one。 But this is a quite impracticable procedure； as it
surpasses all our powers to discover all the possible inferences
that can be drawn from a proposition。 But this mode of reasoning is
employed； under favour； when we wish to prove the truth of an
hypothesis； in which case we admit the truth of the conclusion…
which is supported by analogy… that； if all the inferences we have
drawn and examined agree with the proposition assumed； all other
possible inferences will also agree with it。 But； in this way； an
hypothesis can never be established as a demonstrated truth。 The modus
tollens of reasoning from known inferences to the unknown proposition；
is not only a rigorous； but a very easy mode of proof。 For； if it
can be shown that but one inference from a proposition is false；
then the proposition must itself be false。 Instead； then； of
examining； in an ostensive argument； the whole series of the grounds
on which the truth of a proposition rests； we need only take the
opposite of this proposition； and if one inference from it be false；
then must the opposite be itself false； and； consequently； the
proposition which we wished to prove must be true。
The apagogic method of proof is admissible only in those sciences
where it is impossible to mistake a subjective representation for an
objective cognition。 Where this is possible； it is plain that the
opposite of a given proposition may contradict merely the subjective
conditions of thought； and not the objective cognition； or it may
happen that both propositions contradict each other only under a
subjective condition； which is incorrectly considered to be objective；
and； as the condition is itself false； both propositions may be false；
and it will； consequently； be impossible to conclude the truth of
the one from the falseness of the other。
In mathematics such subreptions are impossible； and it is in this
science； accordingly； that the indirect mode of proof has its true
place。 In the science of nature； where all assertion is based upon
empirical intuition； such subreptions may be guarded against by the
repeated parison of observations； but this mode of proof is of
little value in this sphere of knowledge。 But the transcendental
efforts of pure reason are all made in the sphere of the subjective；
which is the real medium of all dialectical illusion； and thus
reason endeavours； in its premisses； to impose upon us subjective
representations for objective cognitions。 In the transcendental sphere
of pure reason； then； and in the case of synthetical propositions；
it is inadmissible to support a statement by disproving the
counter…statement。 For only two cases are possible； either； the
counter…statement is nothing but the enouncement of the
inconsistency of the opposite opinion with the subjective conditions
of reason； which does not affect the real case （for example； we cannot
prehend the unconditioned necessity of the existence of a being；
and hence every speculative proof of the existence of such a being
must be opposed on subjective grounds； while the possibility of this
being in itself