Any comment on his list? Items you would reformulate, combine, add or eliminate? Is this checklist a good schematic of the challenges facing theoretical physics at this point in history? I find it helps to have memorized so I can use it as an informal gauge of progress and relevance. One thing that helps in memorizing is to notice that there are TWO unifications a unification of laws and a (possibly partial) unification of fields. The unification of laws could be seen as imperative: quantum mechanics and general relativity both describe nature and nature is ONE. There must exist a quantum mechanical version of GR and a general relativistic version of QM, which form an organic whole. At present they appear formally incompatible and limited in their applicability. Both suffer from infinities/singularities and are incomplete. There must be a single theory to replace them (or so the argument goes). But this unification of the basic laws does not logically require that all forces be aspects of a single force, or that all particles be versions of a single particle, or that any such unification of fields must be achieved. That is a different sense of unification and I guess people might risk confusion if they speak as if they equate the two and moosh their unification fantasies together. In any case, whether it is right or wrong, the checklist has two separate unification items: #1 and #3. Once you take account of that, it is easy to assimilate and remember: ==quote== Problem 1: Combine general relativity and quantum theory into a single theory that can claim to be the complete theory of nature. (This is called the problem of quantum gravity.) Problem 2: Resolve the problems in the foundations of quantum mechanics, either by making sense of the theory as it stands or by inventing a new theory that does make sense. Problem 3: Determine whether or not the various particles and forces can be unified in a theory that explains them all as manifestations of a single, fundamental entity. Let us call this problem the unification of the particles and forces, to distinguish it from the unification of laws, the unification we discussed earlier. Problem 4: Explain how the values of the free constants in the standard model of particle physics are chosen in nature. Problem 5: Explain dark matter and dark energy. Or, if they don't exist, determine how and why gravity is modified on large scales. More generally, explain why the constants of the standard model of cosmology, including the dark energy, have the values they do. ==endquote== These quotes are excerpted from pages 5-16 of the first edition of Smolin's book TWP. It's the latter part of Chapter 1, and probably the same pages in the orange paperback edition. I think it might be worthwhile to discuss them, and to see what comment people have.