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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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References: <D2IInB.8o6@gpu.utcc.utoronto.ca> <19JAN95.13317188.0017@ESAMATC.LIB.MATC.EDU> <ONEAL.95Jan19201224@trantor.astro.psu.edu>
Date: Mon, 23 Jan 1995 04:27:18 GMT
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In article <ONEAL.95Jan19201224@trantor.astro.psu.edu> oneal@astro.psu.edu (Doug O'Neal) writes:
>
> Galileo did provide the observations that made the heliocentric universe 
> more likely.  The clincher (in his mind) was that Venus shows phases like 
> those of the moon.  In a geocentric cosmology, Venus would always appear 
> as a crescent, never as gibbous or full.  Galileo's reasoning, however, 
> was something that the learned world wasn't used to: backwards logic, the 
> kind that most science relies on today.  Consider the statement: "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.  It's this kind of model building and relying on likelihoods 
> on which modern science depends; but it was new in his day, and in fact, 
> as a syllogism, it's backwards.  Thus theologians and others were still 
> able to claim, well, there could be another reason for Venus to show 
> complete phases.  

An argument of the form "if P then Q, Q, therefore P" is considered
invalid even now.  It's the fallacy of affirming the consequent.
Moreover, so far as the argument reported above goes, there might
well be other reasons for Venus to show phases.  I don't think 
galileo's reasoning could have been quite that simple.  For
instance, he might argue that the views he was arguing against
would have Venus not showing phases.  So phases them become a
problem for them, but not for him.  Also, Galileo's argument as
you report it above doesn't say anything about likelihoods, so
saying someting about them might be another way to strengthen it.
