Continuing the discussion from Ye olde history of perpetual motion machines:
It's not so much the laws of thermodynamics as the laws of arithmetic. It's established that the energy needed to lift something up is exactly equal to the energy gained by letting it drop back down, minus losses due to friction. (And the same holds true for motion through magnetic fields, etc.) If the laws of arithmetic are correct, then every combination of energy sources and equal energy sinks will always add up to zero.
Perpetual motion machines involve setting up a complicated system with lots of moving parts and then "forgetting" to add in one of the energy sinks.
People get upset about threats to change the laws of arithmetic because they know on a gut level that no matter what changes were made, they'd somehow wind up owing back taxes and mortgage interest because of it.
Now, what would be interesting is a perpetual motion machine based on some kind of exotic matter that's light when it's lifted up, but becomes heavy when it falls down. Nobody ever builds those.