JPM Miao: Why can't R understand if(num!=NA)?
Peter Dalgaard: Because
comparison with an unknown value yields an unknown result.
David Winsemius:
Anything else would violate the Second Law of Thermodynamics. We cannot have
comparisons reducing entropy, now can we? Uncertainty cannot run uphill.

JPM Miao, Peter Dalgaard, and David Winsemius
on why is needed
May 2013