  The Principle of Unattainability rules out the attainment of absolute zero
temperature by any finite physical means, no matter how idealised they could
be. Nevertheless, we clarify that the Third Law of Thermodynamics, as defined
by Nernst's heat theorem statement, is distinct from the Principle of
Unattainability in the sense that the Third Law is mathematically equivalent
only to the unattainability of absolute zero temperature by {\em quasi-static
adiabatic} processes. This, on the one hand, leaves open the possibility of
attainability of absolute zero by non-adiabatic means, without violating the
Third Law. On the other hand, we point out some apparent incompatibility
between the Postulate of Projective Measurement in quantum mechanics and the
Principle of Unattainability in that projective measurements of energy could
result in zero temperature.
