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From: jeff@aiai.ed.ac.uk (Jeff Dalton)
Subject: Re: Heliocentricism (Re: THE PURPOSE OF LIFE Defined & Gaia)
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Date: Wed, 25 Jan 1995 17:54:36 GMT
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In article <3fvtr9$qdg@agate.berkeley.edu> <jerrybro@uclink2.berkeley.edu> writes:
>jeff@aiai.ed.ac.uk (Jeff Dalton) wrote:
>
>> An argument of the form "if P then Q, Q, therefore P" is considered
>> invalid even now.
>
>But not the argument, "if P then Q, Q, so let's suppose P for the
>time being."  This is abduction, and it's used all the time.

It's not clear that that's an argument, let alone a valid one.  It's
true that abduction / inference to the best explanation is often used,
but so is the argument that there could be another reason.  For
instance, compare:

  Thus theologians and others were still able to claim, well, there
  could be another reason for Venus to show complete phases.  [From
  the article I was answering]

and

  Agreed.  But then I was questionning the assumptions leading to UG,
  rather than attempting to prove an alternative view.  In order to
  question those assumptions, it is sufficient to show that other
  possibilities are plausible.  [From a recent article by Neil Rickert]

Now, I don't think Galileo confined himself to the following (formally
invalid) argument: 

  "If the solar system is heliocentric, Venus will show moon-like
  phases".  Galileo observed that Venus shows moon-like phases;
  therefore, the solar system is heliocentric.

-- jd
