[nyrikki]

Chess is decidable, it may be PSPACE-hard or EXPTIME-hard, but there are reductions.

Entscheidungsproblem and Halt are not decidable in the general case.

While you have to find reductions, decidable problems having access to both yes-instances and no-instances makes it easier to find them.

[itishappy]

Knowing that we don't approach either by attempting to fully solve them, does that change the architecture, or just the difficulty?