As we know, the doctrine of parallelism in contemporary philosophy has been combatted on many fronts. Due to the influence of a number of important thinkers who embrace the doctrine of interaction, parallelism has been forced out of the dominant position it long occupied. Now we know that once we agree about the true character of the concept of the physical, all interaction is certainly ruled out. But we can of course seek to retain the notion of interaction if by "physical" we wish to understand something different. This in fact is what is done by the supporters of the notion, often without expressing themselves clearly about the concept of the physical on which they base themselves. For this reason, if for no other, it is useful to examine their arguments; their presuppositions are thus more readily revealed. If we are then able to show that these assumptions are unproved, the attacks on parallelism will have been repulsed and the theory made the more secure.