Compatibilism is the doctrine that determinism and free will are 
compatible. I do not think comaptibilist arguments work because they
employ strange and unpersuasive definitions of free will 
(and, occasionaly physical law). There is a sort of new-lanmps for old scam
being worked; you can have your belief in both the laws of nature and 
human freedom, providing you exchange your current notion of freedom for
the compatibilists one.
 
According to Thomas Hobbes, you are free so long as you are able 
to do what you want:

"Liberty is the absence of all the impediments to action 
that are not contained in the nature and 
intrinsical quality of the agent"

Your freedom may be affected by being in jail, 
but it is not affected by the truth or otherwise
of causal determinism. Even if your actions today were already fixed by the 
physical state of the universe before you were born, you are still free so 
long as you can do what you want.

The truth of this is not obvious -- indeed to some it is obviously false: 
Kant called it a 'wretched subterfuge'. If John were to put James in 
manacles and make him do his bidding, Hobbes and everyone else would 
say that James was not free.

However, if John were to hypnotise James into being a willing and 
eager slave whose only desire was to do John's bidding, Hobbes -- 
but few others -- would say James was free, since he was doing 
what he wanted. Hobbes' theory seems to miss something, 
the ability to *choose* what one wants.

His account also misses the fact that we impute volition only to 
certain entities. We do not say a stone is free simply because 
it is not in a cage, although we might say that of a bird. 
Freedom is not just a negative condition of being free of constraint,
 it is also a positive power which only some entities possess. 

Believers in free will, libertarians, often emphasise two key 
features of free will: alternative possiblities (AP), the 
idea that there is a genuine choice available at
a given time, and ultimate responsibility (UR), AKA origination, the idea
that there is a genuine sense in which you ar the originator of your voluntary 
acts, and therefore bear responsibility for them.

In other words there is a choice to be made, and it is indeed you who are 
repsonsible for making it.

It could be argued that the power to make choices is compatible with 
determinism -- after all computers make choices, and they are deterministic. 
However, we can guarantee that two identically programmed computers will 
always make the same 'choice' under the the same circumstances, and that 
the 'choice' it makes is always predictable given knowledge of its programming 
and external inputs; it brings nothing uniquely of its own to the 'choice'.
By contrast, libetarians believe that, having chosen they could have chosen 
differently under the very same circumstances. That is, that alternative 
possibilites were actually available, they have a positive power of choice, 
since they bring  something of their own to a choice, they have ultimate
responsibility, unlike computers whose 'choices' are always attributable 
to external and prior factors.  We cannot attribute UR to computers
because one is always substitutable for  another, the individuality of 
the computer makes no difference.

Compatiblists sometimes argue that Alternative Possibilites are available
in a deterministic universe. They claim that because E happens at time T, 
it did not have to happen; something else, E' ws possible sot there were 
AP's. E does not have to happen, because its cause did not have to happen 
(and so on back to the Big Bang).

This is another wrteched subterfuge, though.
Once we get to the point whee C has occurred, then E must occur, if determinism
holds. Undetr the circumstances under which I find myself at time T, only
E is possible. (i.e. ther is a difference between possibility per se -- 
according to which E' is possible -- and possibility-under-the-circumstances 
according to which it is not).

The compatibilists version of Alternative Possibilities is is really 
alternative histories. It says the hsitory of the universe could have been 
different up to time T.  However, I have no power to change history, 
so the Compatibilist version of Free will doe snot add up to any 
positive power of volition.

Peter D. Jones 3/8/01