[archangel_one]

It's not that theoretical, the following complex program fits that description:

def IsProgramAVirus(program): return True

But it's not particularly useful. In practice the false positive rate is extremely important, because if it gets too annoying users will switch it off.

[terangdom]

In the best of worlds, developers could make an effort not to do anything shady, like self modifying code, dll-injection etc, making heuristics complaining about such behaviours viable.