Accordingly the judgments of the common sense which chiefly interested Reid are what he calls first principles of ‘contingent’ truths, as distinguished from abstract necessities the opposites of which are self-contradictory or unthinkable. Contingent truths, on the other hand, may be rejected in thought; but those who reject them speculatively must proceed upon them in their actions and reasonings, including even sceptical reasonings. The man whose scepticism involves a practical surrender of the common judgment, that what we call the outward world is independent enough of _his_ individual existence to make it a trustworthy medium of intercourse for him with other living persons; and who acts accordingly on the belief that ‘other persons’ are only conscious states of his own individual mind, would be pronounced a lunatic. Again, the fatalist, who rejects practically the moral judgment that refers the issues of a deliberate voluntary determination to the voluntary or personal agent as its responsible centre, insanely sits still in the midst of danger, and refrains from exerting a power which he denies. And he who refuses to proceed upon the logically undemonstrable postulate of universal natural order, by ceasing to reason about wholly uninterpretable chaos, is inevitably crushed by the Universal Power that he ignores. It is thus that practical disregard of inspired final reason appears to Reid to be ‘destructive at once of the science of the philosopher, the prudence of the man of the world, and the faith of the Christian.’ The unjust as well as the just, so far as they live at all, must, he sees, live by faith in what cannot be either proved or disproved by direct demonstration. And if all those final judgments of practical reason were contradicted in daily action, as well as in academic theory, ‘piety, patriotism, parental affection, and private virtue would appear as ridiculous as knight-errantry: the pursuits of pleasure, ambition, and avarice must be grounded upon this sort of belief as well as those that are honourable and virtuous.’