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Full Idea
By determinism I mean that the prevailing laws of nature are such that there do not exist any two possible worlds which are exactly alike up to that time, which differ thereafter, and in which those laws are never violated.
Gist of Idea
Determinism says there can't be two identical worlds up to a time, with identical laws, which then differ
Source
David Lewis (Causation [1973], p.196)
Book Ref
'Causation', ed/tr. Sosa,E. /Tooley,M. [OUP 1993], p.196
A Reaction
This would mean that the only way an action of free will could creep in would be if it accepted being a 'violation' of the laws of nature. Fans of free will would probably prefer to call it a 'natural' phenomenon. I'm with Lewis.