Peter Jones writes:

> A computation is not a series of states. A computation is an
> implementation
> of an algorithm, and algorithms include conditional statements which
> must be modelled by something with counterfactual behaviour --
> by something which *could have* execute the other branch.

Whatever else a computation is, it is a series of states. My computer 
is going through a series of physical states, with the earlier states 
determining the later states. If the earlier states were different, then 
the later states would also be different, hence the computer handles 
counterfactuals. However, this is so with any physical system: it goes 
through a series of states, the earlier states determine the later states 
following the laws of physics, and had the earlier states been different, 
so would the later states. Now, I suppose you would say that the states 
in a rock are random, while those in a computer are not. But what is to 
stop someone from designing a computer so that there is no pattern to 
its internal states unless you have the key? Suppose you find two 
inputless electronic devices, powered up, with complex and at first glance 
random currents circulating in their internal components. One of these 
devices is in fact implementing a computation, deliberately scrambled 
to keep it secret from prying eyes, while the other is just a decoy with 
random electrical activity. Without access to the key, would you be able 
to tell which is which?

Another question: I can see why a computer should be able to handle 
counterfactuals if it is to be of practical use, but what is wrong with 
saying that a recording implements a computation, whether that is 
adding two numbers or having a conscious experience? 

Stathis Papaioannou
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