[extropy-chat] what is probability?

gts gts_2000 at yahoo.com
Thu Jan 11 16:20:02 UTC 2007

Where oh where have the true subjective probabilists
gone? Do they exist? What have they to say for

For Christmas, Santa gave me a copy of _Subjective
Probability: The Real Thing_ by professor Richard
Jeffrey of Princeton (probably because I was bad in

Surely, I thought, this Jeffrey fellow must be a
hardcore subjectivist to have written a book by such a
name. However on perusing his book it's not
immediately clear to me that Jeffrey does not also
admit to something like objective probability.

On page 19, he writes: 

"1.8 OBJECTIVE CHANCE. It is natural to think there is
such a thing as real or objective probability
('chance', for short) in contrast to merely judgmental

I've observed that when subjectivist probabilists like
Jeffrey refer to what objectivists seem to mean by
'objective probability' or 'objective propensity',
they substitute some other term such as 'objective
chance'. Is this a case of a rose by any other name?

Jeffery does however offer what seems to be a
subjectivist interpretation of objective chance: 

"David Hume's skeptical answer to those questions says
that chances are simply projections of ROBUST features
of judgmental probabilities from our minds out into
the world, whence we hear them clamoring to be let
back in."

Ahh, now there is some real philosophy. This is the
sort of thing I was hoping to read about. I wonder if
anyone here is familiar enough with Hume to offer a
clear definition of what he meant by a "robust" (vs a
non-robust) feature of judgmental probability. 


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