"That Marx was truly and honestly convinced of the truth of his thesis I do not doubt. But the grounds of his conviction are not those which he gives in his system. They were in reality opinions rather than thoughtout conclusions. Above all they were opinions derived from authority. Smith and Ricardo, the great authorities, as was then at least believed, had taught the same doctrine. They had not proved it any more than Marx. They had only postulated it from certain general confused impressions. But they explicitly contradicted it when they examined things more closely and in quarters where a closer examination could not be avoided."