 From within Wharton’s picture, an invariant joint probability distribution associated with each possible pair of initial and final conditions can be constructed, and the usual conditional probabilities can be formed by conditioning on any chosen portion of the boundary (Wharton 2010a,b). Probability is then a manifestation of our ignorance: if one knew only the initial boundary, one would only be able to describe the subsequent field probabilistically (due to the future constraint); given the final boundary as well, one would then be able to retrodict the field values between the two boundaries.