Emergence and computationally irreducibility, Prof. Rees 'building' metaphor is fascinating to understanding how humans peel off layers of reality from Nature's big onion. Thanks Lex. The key issue seems to be whether one can jump from layer to layer, which strong emergence claims is definitely not possible. So from physics to biology, from biology to psychology, etc,etc.... However, would be great hearing from detractors of Dark energy Dark matter, there are very credible hypothesis and laureate scientist who claim GR is wrong and we have invented Dark energy and dark matter to fill in its explanatory deficiencies. If we find a new modified theory of Gravity then we don't need Dark energy or matter. Same thing happened with Planet Vulcan in the early 1900s. Scientists invented this invisible planet to explain the odd precesion of Mercury' orbit perihelion, turns out GR could explain it without the extra planet so Vulcan was thrashed. I am a physicist and do not buy into Dark matter nor Dark energy, but rather in Moffat's and Milgrom's MOND. After all no one has ever seen nor detected any Dark energy/matter, and my guess is that it will also be thrashed once a new GRv2 is invented.