Emergent properties is a phrase that I find difficult to pin down. This is not an unusual thing with words; many words are ambiguous in many useful ways. Probably all words have some level of ambiguity. However when I encounter emergent properties I get a feeling of something that can plainly be said is being avoided. It seems a slippery phrase meant to hide meaning. Of course, this is my reaction and not necessarily the author’s intent. The author may have a particular ambiguity in mind and feels he is being open about it. Unfortunately, my reaction is to immediately distrust the author and to look for what they are hiding.
Emergent properties may be as mundane as ‘the whole is greater than the sum of its parts’, which is true of many (maybe most) things and hardly worth mentioning. It may be being used to say, ‘this is understandable at a level higher than elementary physics, say thermodynamics or biology or meteorology’. Again this is a mundane observation as everything except elementary physics is understood at a higher level than elementary physics. Or it may perhaps mean that a thing is not reducible to the nature of its components. If the thing is in principle reducible but we do not actually ever bother to do the reduction then it is the same mundane statement. But if it means that the thing is in principle not reducible then it may still be mundane if the phrase is intended to mean, ‘by the nature of quantum mechanics, mathematical calculations and other restraints it is not in principle possible to actually reduce macroscopic systems to the behavior of their elementary particles even though the particles cause macroscopic properties’. Such mundane meanings hardly ever need to be stated (they are assumed) and if there is a need, than they can be clearly said.
On the other hand emergent properties can have a meaning that is anything but mundane. The phrase may imply upward causation. This is an odd idea – the whole causes behavior of the parts. For example, there is something unusual about the properties of consciousness that cause the properties of the parts of the brain (neurons, proteins, atoms etc.) to be what they are. Or the phrase may imply no material causation at all. For example, there can always or occasionally be no causal connection between the mind and the brain. These uses of emergent step outside of a physical/material universe and introduce a mysterious other substance/energy that is unknown, magic, spiritual or something – who knows.
So when I encounter someone referring to an emergent property, I am left thinking which is it: someone with conventional notions of the physical world but is leaving open the possibility that the reader can think he believes otherwise; or is it someone who does not have conventional notions of the physical world but is leaving open the possibility that the reader can think he does. Or maybe it is someone who thinks that the phrase sounds learned and is not saying or implying anything at all. I really try to be more generous and understanding but I find it very difficult. Emergent properties just pushes my buttons.