Originally posted by etenguy
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There is set of circumstances or evidence (E) that make it look like Parry might have done it. E is basically an "inside job" and includes: involving the cashbox, knowing his chess club schedule, Julia not admitting strangers, and so on. One possibility is that a guilty Wallace set up E to frame Parry. However, E allows an innocent Wallace to infer that Parry did it. The logic on this seems pretty unassailable to me.
My initial argument was a reductio ad absurdum:
(P) Wallace carefully planned to frame Parry (because of E)
and
(1) Wallace could not know whether Parry had an alibi or not
(2) If Parry had an alibi, E makes it appear that no one else could commit the crime except Wallace
I don't think anyone really disputes (1) and (2) - there might be a few skirmishes on detail, that's all.
But
(3) For Wallace to have planned this particular murder scheme (it needed much thought), he would have seen both (1) and (2) and the dangerous position it would leave him in (facing a murder charge with the death penalty)
Therefore
(P*) Wallace would not have carefully planned to frame Parry (he would have devised some other method, e.g. poisoning).
Hence, from (P) we get its negation (P*) and we must reject (P). This is the argument in its simplest form. I suggest it provides a rational foundation for accepting (b) from earlier. Obviously, other evidence and arguments need to be considered, too. But if you ultimately reject (b), I agree with you that you should accept (a) rather than simply that "Wallace was guilty".
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