  The solution of the linear transport equation used for the study of neutral
particle fields requires the imposition of appropriate boundary conditions. The
choice of the conditions to impose for an infinite medium is not
straightforward. The question has been given different formulations in the
literature with various justifications based on some physical reasoning. Some
aspects of the question are here analysed, from both the mathematical and the
physical point of view. It is concluded that the inspiring golden rule should
be the establishment of conditions that do not require any reference to the
properties of the specific medium being considered for their justification.
