On 13-Jul-02, Hal Finney wrote:
> Causality is a fascinating topic and one I hope to learn
> more about, perhaps via the Pearl book Tim mentioned or
> another that someone might recommend. Here are some very
> random and disorganized thoughts on the topic.

> Googling on "causality" led to a brief web page that
> summarized 2000 years of philosophical thought in a few
> paragraphs,
> http://www.helsinki.fi/~mqsalo/documents/causalit.htm.
> Generally there seem to be two schools of thought; one is
> that causality is an artifact of our minds' efforts to
> understand and interpret the world; the other is that
> causality has objective reality.

> The modern philosopher's definition of causality has
> always struck me as somewhat backwards: "The cause of any
> event is a preceding event without which the event in
> question would not have occurred." This means that "A
> causes B" is equivalent to A happening before B, plus the
> logical proposition, "if B then A". You'd think that A
> causes B would mean "if A then B". But philosophers
> interpret it to mean "if not-A then not-B", as stated
> above (I've read this elsewhere also), which is equivalent
> to "if B then A".

> So if we say "heavy rains cause flooding" we mean that if
> there were no heavy rains then there would be no flooding;
> or equivalently, if there is flooding then there must have
> been heavy rains. This is consistent with there being
> heavy rains and no flooding. It really doesn't make sense
> to me.

Such definitions of "causality" seem to miss the point that
every event has mulitple causes and constraints and that
any one of them is neither necessary nor sufficient.  As
commonly used "the cause" is something(s) we consider could
have been otherwise.  Heavy rains may be rare and so it is
easy to suppose their absence.  In that case we would say
heavy rains caused the flooding.  But if you lived where
heavy rains were the rule, you'd more likely say something
like, "The flooding was caused by insufficient foresight in
providing drainage."
> The real mystery is why our physical laws are time
> symmetric. A priori it does not seem to be absolutely
> necessary for life to evolve. I mean, of all sets of
> physical laws that allow life to evolve, will most be time
> symmetric? I'm not aware of any reason why this should be
> the case.

Some physicists (Hartle, Penrose, Gell-Mann) regard quantum
mechanics as defining an arrow of time more basic than the
statistical increase of entropy.  This QM AoT is defined by
the decoherence or 'collapse of the wave function'.  I
suppose the splitting of MW would be the same. However,
Bohm's QM is deterministic and doesn't have this QM AoT
even though it gives the same predictions as
non-relativistic QM. 

> This elevates causality to having a functional role in the
> physical world! It makes the difference between a system
> having a mind and just being an elaborate tape recorder.
> It means that we cannot view causality as just an artifact
> of our perceptions, it must be a true element of reality.
> That would put significant constraints on how philosophers
> treat the concept of causality.

I don't see how a concept of causality can solve this
conundrum.  The 'computation' can be causal and yet so
simple that the mapping still does all the work.  I think
the answer lies in embedding computation within an
environment.  The reason the mapping can apparently do all
the work is because it is all at a symbolic level.  If you
imagine building a robot, the the 'brain' of the robot must
do computation taking information from sensors and
producing signals to actuators. Then it seems the question
of whether the robot has done a computation has operational
meaning and the conundrum doesn't arise. 

Brent Meeker
There are two ways to write error-free programs.
Only the third one works.

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