 While the time asymmetry challenge can be raised for the Green’s function associated with hyperbolic equations, it is worth pointing out that there are also theories or theoretical frameworks in physics with time-asymmetric Green’s functions, chiefly among them linear response theory. There is no philosophical consensus on the causal status of the Green’s function formalism. Frisch (2009a; 2009b; 2014) argues for a causal interpretation of the Green’s function formalism, both in the case of the time-reversal invariant wave equation and in the case of the explicitly time-asymmetric linear response theory, while Norton (2009) and Smith (2013) are critical of causal interpretations of the formalism.