Below is a technical refutation of determinism by Alvin Plantinga taken from his recent book Science, Religion, & Naturalism. Where the Conflict Really Lies. He shows that determinism is necessarily false. He starts with a premise about the laws of nature which states that the laws of nature are as they are provided God doesn't act to change them. Of course you do not need to believe in God in order to believe this because it is a conditional statement that doesn't presuppose the existence of God. He then goes on to assume the truth of determinism, and then deduce a falsehood. Given that determinism entails a falsehood, determinism must be false.
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(LN) When the universe is causally closed (when God is not acting specially in the world), P.
For example, Newton's law of gravity would go as follows:
(G) When the universe is causally closed, any two material objects attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.
Note that on (LN), the currently canonical account of determinism implies that determinism is false. According to that account, determinism holds just if the natural laws conjoined with the state of the universe at any one time entails the state of the universe at any other time. A bit more exactly: let ''L'' be the conjunction of the natural laws, and S(t) and S(t*) be the states of the universe at any times t and t* : then,
Necessarily, for any t and t*, if L & S(t), then S(t*).
It is worth noting that if the above account of natural laws is correct, determinism so understood is false and indeed necessarily false. For suppose determinism is true. According to (LN), a natural law is of the form
If the universe (U) is causally closed, the P.
Take the conjunction of the natural laws to be
If U is causally closed, then P,
where now P is the conjunction of the consequents of all the laws. Let ''PAST'' denote a specific past state of the universe. Now suppose determinism is true. Then
(1) (If U is causally closed, then P) and PAST
entails
F (the actual future)
that is, (using 'N' to mean 'Necessarily'),
(2) N (If (1) then F).
(2) is equivalent to
(3) N [if (if U is causally closed, then P) and PAST, then F],
that is,
(4) N [if (either U is not causally closed or P) and PAST, then F],
that is,
(5) N {if [(PAST and P) or (PAST and U is not causally closed)] then F}
(5) is of the form
N if (p or q) then r;
but then each of p and q entail r; hence
(6) N [if (PAST and P) then F] and N [if (PAST and U is not causally closed) then F].
But the right hand conjunct of (6) is obviously false: clearly there is a possible world that (i) shares its past with the actual world, (ii) is not causally closed (because, perhaps, God acts specially in it) and (iii) does not share its future with the actual world. Therefore determinism, which entails (6), is false. Indeed, given the usual view that propositions of the form necessarily p are non-contingent, either necessarily true or necessarily false, (6) is necessarily false; hence determinism, which entails it, is also necessarily false.