is correct it would seem to create an irony for other commentators who take spontaneous symmetry breaking to undermine Curie' Principle (see Radicati s 1987). Part of the di culty here is due to the facts that there is no canonical de…nition of spontaneous symmetry breaking and that this notion is used in di¤erent ways in di¤erent contexts. At least three di¤erent strands to the discussion of this concept can be distinguished. The …rst and, perhaps, most fundamental strand leads to cases where there is a symmetry of the equations of motion or …eld equations of a system that is not a symmetry of a state of the system that is of special physical 3 signi…cance.2 Without the quali…er "of special physical signi…cance," cases of spontaneous symmetry breaking would be all too easy to …nd. That a law of motion/…eld equation obeys a symmetry principle– say, time reversal invariance or invariance under spatial rotations– does not imply that a particular solution, or a particular state belonging to the solution, exhibits the symmetry at issue. Indeed, the solutions states exhibiting the symmetry at issue may be the exception rather than the rule. For example, Einstein' gravitational …eld equations are time reversal invariant, but the set of s Friedman-Walker-Robertson cosmological solutions that are time symmetric about some time slice are of "measure zero" (see Castagnino et al. 2003). And one would guess that a similar "measure zero" result would hold for rotationally symmetric states in a classical or quantum theory with laws of motion that are invariant under spatial rotations. I take it that this is the kind of point Curie was making when he wrote that "it is asymmetry that creates phenomena" (Curie 1894, p. 400).4 But if it is true that the symmetries so beloved by physicists are typically broken by the phenomena we actually observe, one can wonder about the warrant for setting such store by these symmetries (see Kosso 2000). This issue will not be treated here since my focus is on issues in the foundations of physics rather than on the epistemology and methodology of science. What cannot be avoided here is the issue of how to give content to the quali…cation "of special physical signi…cance," since without a speci…cation of content "spontaneous symmetry breaking" does not point to any de…nite set of phenomena requiring special treatment. While the physics literature on spontaneous symmetry breaking does not show unanimity, the focus tends to be on ground states, or vacuum states, or equilibrium states at a de…nite temperature, states which one might expect should exhibit a symmetry of the basic physical laws governing them.5 This leads to the second strand of the discussion. It is often said that cases of spontaneous symmetry breaking involve de2

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