Abstract
Tony Leggett has suggested that quantum theory cannot be applied to complex macroscopic objects. This essay supports that idea by giving two specific examples of complex systems where this is true, because of the essential nature of the quantum measurement process, which cannot be described by standard quantum theory (none of the alternatives proposed in the end get round this limitation, in practical terms). I then place this result in the larger context of the ubiquitous occurrence of top-down causality in complex systems, which is the key process whereby genuine complexity emerges from the underlying physics. The implication is that the ability of physics to comprehend the dynamics of complex systems, such as life, is strictly limited: physics underlies and strongly constrains what happens, but in the end does not determine the unique outcome that actually occurs. This is determined by autonomous emergent higher level dynamics.
George F. R. Ellis