A distinction that makes all the difference in the world.
The following extremely cogent excerpt is thanks to Agent Intellect.
Now the question is, does this counter-argument of O’Hear’s determinist enable him to escape Popper’s charge that asserting physical determinism to be true is self-defeating? Surely not. For let us suppose that there are three people, A, B and C, who are ‘determined in such a way that they accept only those positions which are reasonably argued’ and that the rest of us, D . . . N, are not such people.
Now, could D pick out A, say, as belonging to the former group? Could D be sure that A accepted only those positions which are reasonably argued? Evidently not, for in order to be sure of this he would have to be able to check to see whether the positions A accepts are reasonably argued. But by hypothesis he cannot do this. And he could not do this for B and C either. Hence A, B, and C’s acceptance of certain positions, even of only those that are reasonably argued, would be of no use as a guide to truth or reasonableness for D . . . N. But more importantly, could A, B, and C themselves be sure that they were among the favoured few? No, for even though they might accept only those positions which are reasonably argued, they would accept them not because of their perception of the reasonableness of the arguments on which they were based, but because of various physical causes. In other words, they could no more check to see whether their positions were reasonably argued than could any of D . . . N. And as for D . . . N themselves, they might be determined to accept positions for which no reasonable arguments could be provided — including the position that the positions which A, B, and C accept are not reasonably argued! — while yet being determined to believe the contrary. Thus on O’Hear’s suggestion no one could ever tell whether any position anybody accepted was reasonably argued; and, in particular, no one could tell whether physical determinism was reasonably argued and, further, whether it was true.
But there is a further problem. The suggestion we are considering is that some people might be determined in such a way that they accepted only those positions ‘which are reasonably argued’. Does this imply that there is some being who reasonably argues for certain positions, and that the people referred to accept only those positions? But who could that being be? If it itself is a being that is determined to accept only positions ‘which are reasonably argued’, then we get involved in an infinite regress. If it is actually capable of apprehending the reasonableness of arguments, and accepts certain positions for that reason, then evidently it is not determined in the same way as is envisaged by the physical determinist: it is not one of those people who are determined by physical causes to accept only those positions which are reasonably argued. At any rate, O’Hear is suggesting that even given the truth of physical determinism, it is still possible to decide that there are certain positions that are ‘reasonably argued’ but he leaves unanswered the question who is in a position to decide this. (One suspects that O’Hear is imaginatively viewing a closed determinist physical system from the outside and himself as the being who is capable of deciding which positions held in that system are reasonably argued. But of course if such a relationship vis-à-vis a closed determinist physical system actually obtained, then universal physical determinism would not be true.)