 Symmetry breaking raises a number of philosophical issues. Some of them relate only to the breaking of specific types of symmetries, such as the issue of the significance of parity violation for the problem of the nature of space (see  Section 2.4,  above). Others, for example the connection between symmetry breaking and observability, are particular aspects of the general issue concerning the status and significance of physical symmetries, but in the case of SSB they take on a stronger force: what is the epistemological status of a theory based on “hidden” symmetries and SSB? Given that what we directly observe — the physical situation, the phenomenon — is asymmetric, what is the evidence for the “underlying” symmetry? On this point, see for example Morrison (2003) and Kosso (2000). In the absence of direct empirical evidence, the above question then becomes whether and how far the predictive and explanatory power of theories based on SSB provides good reasons for believing in the existence of the hidden symmetries. Finally, there are issues raised by the motivation for, and role of, SSB. See for example Earman (2003a), using the algebraic formulation of QFT to explain SSB; for further philosophical discussions on SBB in QFT in the algebraic approach, see Ruetsche (2011), Fraser (2012), and references therein. Landsman (2013) discusses the issue whether SBB in infinite quantum systems can be seen as an example of asymptotic emergence in physics.