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.

The problem with judging causality is the well known cautionary rule
that "correlation is not causation".  Just because A and B always
occur together, and A comes before B, that doesn't mean that A causes B.
This is a common danger that scientific researchers have to guard against.
It's easy to observe correlation but hard to determine what is the
true cause.  This is a reason to view causality as just a matter of our
perceptions.

A strange aspect of causality is that to a large degree our laws of
physics appear to be time symmetric.  This means that you can reverse the
time sequence of events and get a physically valid (although possibly
unlikely) scenario.  If that is so, then if A causes B in the forward
direction, can we with equal validity say that B causes A when we look
at things backwards?  In that case the causal roles are fundamentally
arbitrary.

Of course in most situations we have a strong arrow of time based on
the growth of entropy which allows us to break this apparent symmetry.
But this is not always the case.  Systems in thermodynamic equilibrium,
if considered in isolation, have no arrow of time.  We can look at a
microscopic part of such a system and see fluctuations which appear to
be describable in causal terms.  For example, a temporary void forms
randomly, causing matter at the edges to move towards the center.
Or in reverse we say that matter moved away from the center, causing a
void to form.  Both are equally valid ways of describing the situation,
indicating that there is no true causality.

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.

One final point regarding causality.  Although it seems to be more
a matter of metaphysics than physics, there are some places where
people take causality to be a fundamental part of physical reality.
Specifically, this is done in some theories of consciousness.

Functionalists adopt essentially the view that Bruno calls COMP,
that consciousness is a computational phenomenon and any system which
implements the corresponding computation will be conscious.  This is
the foundation for ideas of "uploading", that is, brain simulation by
computer.  Functionalists believe such simulations are conscious in the
same way that the brain was, provided that the simulation is performing
the same computation that the brain did.

This then raises the question of whether a particular physical system
(like a computer) is implementing a particular computation (which is
an abstract program, perhaps expressed in some computing language).
Surprisingly, this is a difficult and yet unsolved question in philosophy.
No one has ever come up with a widely accepted, hard and fast rule or
procedure to decide whether a physical system is implementing a given
computation.

Broadly speaking, the basic approach is to set up a mapping or
correspondence between aspects of the physical system and aspects
of the abstract computation.  For example, with today's computers
the registers and memory elements work by holding electrical charge.
One could map these charge concentrations to values of variables in
the abstract representation of the computation and there would be a
good correspondence.

This works well in this case, but there are two problems.  The first is
that it does not capture the causal nature of the program.  A computer
which was not actually computing, but just playing back a recorded pattern
of charge distributions, would successfully satisfy the mapping.  I'll say
more about this in a moment.  The second problem is that by making the
mapping more elaborate, one can show that simple, even trivial systems
are implementing complex computations; essentially you do all the work
in the mapping.  It's hard to set up a concrete rule for how complex
the mapping can be, and I don't believe anyone has succeeded in this yet.

But the more relevant problem for now is the issue of causality.
Most functionalist philosophers believe that for a physical system to be
truly implementing a computation, it must include all of the causality
of that computation and not just "passively" reproduce the patterns.
When we apply this rule to the functionalist model of consciousness, it
works out like this: if a system has the right causality, it is conscious;
if it is only passive, it is not conscious.

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.

Anyway, these are some of the issues and musings which make causality
of great interest to me.  I hope to learn more about it in the next
few years.

Hal Finney

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