A functional program is an a self-contained expression -- an isolated system following its own rules. The foremost example we have of such a thing is the universe itself, but the universe is not a good example in this discussion, because we have plenty of reasons to think that the universe contains pure (not pseudo-) randomness. Beyond that, isolation , when it matters, is not an easily proven proposition, and is a deplorable fantasy when assumed in many of the other science and engineering disciplines.