 The time-asymmetry challenge is sometimes raised in the context of discussing the interpretation of a theory’s so-called “Green’s function”, which specifies how a system responds to a localized point-like disturbance. For example, the Green’s function associated with the wave equation describes the circular ripples on a pond’s surface after a small pebble was dropped in. When a theory’s equations are linear, the overall response of a system to multiple point-like disturbances can be calculated by summing or integrating over all disturbances. The important mathematical result is that any solution to the theory’s equations can be represented as a sum of two components: a sum or integral over the Green’s function for all the point-like disturbances plus a solution to the source-free equations. That is, the most general solution to the wave equation applied to our pond will consist of sums of ripples associated with any pebbles being dropped into the pond plus source-free waves on the pond not associated with any pebble as their “source”.