While I think that Turing's prosecution was absurd and inhumane, I'm not sure it's an easy step to conclude that he should be retroactively pardoned now that the relevant laws have changed.
To do so would be to accept that there is no definitive law at any present moment, and run the risk that individuals could be convicted in the future for activities which were perfectly legal at the time. This could have dire implications for both justice and personal liberty.
Your conclusion does not hold: making something retroactively legal is vastly different than making something retroactively illegal. The first happens all the time and is the entire point of a full pardon. In the US the latter is unconstitutional and the former explicitly constitutional.
Besides which, your approach assumes the law that allows for chemical castration because he was gay was a legitimate application of state power. I maintain that it was not.
Exactly. The law in any sufficiently just system is always asymmetric with respect to guilt and innocence, in innocence's favour. Retroactive action should be no difference.
> ... making something retroactively legal is vastly different than making something retroactively illegal.
I can see the difference. But the similarity is (as I pointed out) that accepting either of them makes the case for the other stronger by removing a barrier - that barrier being the presumption that the law is fixed at a particular point in time.
> The first happens all the time and is the entire point of a full pardon.
I can't think of any examples of illegal acts being made retroactively legal (and certainly not "all the time"). Could you cite some examples for my benefit?
I think you're wrong about what a pardon is. A pardon is a way of publicly acknowledging that a (guilty) individual has fully repaid their debt to society. It's used explicitly to release people who have undoubtedly broken the law from the consequent obligations (i.e. prison/gallows) and is done at the behest of monarchs or their governments.
Pardons are a very thorny issue in general because they do not respect the notion that all individuals should be equal under the law. It would be unfair for a convicted murderer to escape the noose on the whim of a King (for example) while another murderer is left to hang. Similarly, pardoning Turing would not be fair on all the others who suffered under the same laws.
> your approach assumes the law that allows for chemical castration because he was gay was a legitimate application of state power.
On a loosely similar note, many (some?) countries can prosecute their citizens if they break law outside of the country, despite "it" being legal there. So in theory, you can be put to jail for possessing marijuana when visiting Netherlands. The "in theory" is important as there is no one that could report it back to your home country. I believe this exists to prevent from import of polygamy and such, but it's still interesting.
To do so would be to accept that there is no definitive law at any present moment, and run the risk that individuals could be convicted in the future for activities which were perfectly legal at the time. This could have dire implications for both justice and personal liberty.