That is not how to argue rationally.
Originally Posted by nottaprettygal
Suppose that CaptainChick argues P, and you ask captain chick to back up P. There are two ways which CaptainChick might try and back up P. First, she might try and derive some consequence from P, let us say Q, "if P then Q". However, in what sense does Q back up P, if Q follows from P? Take the following argument:
(If P then Q) If all cats are black then Shade is black.
(Q) Shade is black
(P) All cats are black
This is invalid i.e. the truth of the conclusion does not follow from the premises. Q does not "back up" P if Q is a consequence of P. The only way which this argument can be valid is if P is equal to Q i.e. if P then P. But who considers P a good reason for P? It is impossible for CaptainChick to avoid your charge like this, so what is the alternative?
Perhaps CaptainChick could propose R, from which P is derived i.e. CaptainChick could "back up" or justify P, using R. This is logically valid, as demonstrated by the following argument:
(If R then P) If all cats are black then Shade is black.
(R) all cats are black
(P) Shade is black
However, since P is the proposition which is being argued, and ~P (not-P) implies ~R (not-R) via modus tollens, then nobody who does not already agree with R is going to see this argument as "backing up" P. If the logical content i.e. set of logical consequences of P is just a subset of the logical content of R, then this argument is shamelessly circular, and equivalent to "R & P, therefore P".
There is no logical method of "backing up" arguments. (This doesn't mean that argument cannot be valid and true, only that they cannot be "backed up", justified, or whatever).