 The laws governing gravity, electromagnetism, and the strong interaction are invariant with respect to C, P and T independently. However, in 1956 T. D. Lee and C. N. Yang pointed out that β-decay, governed by the weak interaction, had not yet been tested for invariance under P. Soon afterwards C. S. Wu and her colleagues performed an experiment showing that the weak interaction violates parity. Nevertheless, β-decay respects the combination of C and P as a symmetry. In 1964, however, CP was found to be violated in weak interaction by Cronin and Fitch in an experiment involving K-mesons. This implied, in virtue of the CPT theorem, the violation of T-symmetry as well; since then, there have been direct observations of T-symmetry violation, as reported by e.g., CPLEAR Collaboration (1998). For a careful analysis of the underlying assumptions working in the assessment of T-violation see Roberts (2015) and Ashtekar (2015). The philosophical puzzle is how we can come to know this time asymmetry, given that we cannot truly ‘reverse’ time. Roberts proposes three templates for explaining how this is possible, drawing in particular on a version of Curie’s principle. By contrast, Gołosz (2016) argues that this asymmetry should be understood as applying to a physical process, rather than to time itself. A yet more basic question is that of what time reversal symmetry actually means. On the philosophical debate on this point, see Roberts (2017) and references therein.