Reid, Thomas (1710–1796)

8. Common sense and first principles

Reid is not the first philosopher to have argued that since a view is contrary to common sense it should be rejected. But he is among the earliest post-medievals to reflect systematically on that argument (1785:VI). Common sense is that degree of judgment which is common to those members of humankind with whom we can converse and transact business. The same degree of understanding makes one capable of discovering what is true and what is false in matters that are self-evident. What is contrary to common sense is contrary to what is self-evident.

Sometimes Reid’s characterizations of self-evidence are fairly neutral. Thus whenever we examine the evidence of any proposition either we find it self-evident, or it rests upon one or more propositions that support it, which, in turn, rest upon propositions that support them. But we cannot go back in this way to infinity. Clearly this can only come to a halt when we reach propositions which support all that are built upon them but are not themselves supported. Sometimes his characterizations serve as marks of self-evidence. One mark is that the principle be expressed in a proposition which is no sooner understood than believed. This runs the risk of fielding only trifling propositions. Another mark is that of an opinion’s appearing so early in the minds of people that it cannot be the effect of education or reasoning. But this applies to a great number of beliefs that are simply false, such as the belief that parents know everything that the child wants to know, as well as to others that are true but need justification. Another mark is universality of an opinion, revealed by the tenor of human conduct. This, Reid thinks, will apply to such opinions as the existence of a material world and that there is a right and a wrong in human conduct, and no doubt many others. Another is the indispensability of the belief to our daily conduct of affairs. Yet another is the ridiculous as applied to the contraries of first principles. And, arguably, by the neutral criterion all the deliverances of the senses and of memory are self-evident, although Reid allows that the faculties in question are fallible. Clearly it is far from obvious that just any person with common sense is competent effectively to employ all of these criteria, although that is not to express doubts about their capacity to acquire the competence.

Reid is inclined to withhold the title of ‘axiom’ from the individual deliverances of the senses since philosophers call only necessary truths axioms. He divides his candidates for first principles into first principles of necessary truths and first principles of contingent truths. Such principles appear to serve as starting points from which other truths can be derived – necessary truths from the former kind and contingent truths from the latter. But it is not Reid’s view that necessarily true first principles are intrinsically more certain or more surely known than contingently true first principles. Indeed Reid argues (1785: VII) that probable evidence of the highest degree – consisting of an accumulation of different arguments based on first principles of contingent truths – often leads to a degree of assurance as great as fits a proposition of Euclid.

The first principles of contingent truths concerning the reliability of the senses and of memory, of which Reid offers as direct an account as that (1785: III) for the senses, are almost immediate consequences of the principle that our faculties – senses, memory, consciousness and reasoning – are not fallacious.

In reply to a sceptic who objects that we are prone to error, Reid contends that to claim that our faculties are not fallacious is not to say that they are infallible; only by using them can we establish anything at all, including their proneness to error. But that our faculties are not fallacious is not something that can be established by using them; it is rather a presupposition of their use.

Another sceptic might resolve to withhold assent to anything until it is established that our faculties are not fallacious. Reid remarks that it would be impossible to move them out of this position by argument. Indeed, it is not possible to enter into a dispute with someone who does not share our first principles. For the procedure by which the truth of a proposition is discovered, or its falsehood detected, is to show its necessary connection with first principles or its incompatibility with them; hence one cannot reason with someone who denies one’s first principles. Indeed, if many key terms are implicitly defined by acceptance of first and derived principles it is hard to see how we can even communicate with someone who does not share our first principles.

It might be thought that Reid’s view that first principles are the voice of God should be taken as a possible justification for adhering to them, or for thinking them true. But if it is not self-evident that they are from God (even if it be evident that their acceptance is, in some instances, a part of human nature), it requires use of our faculties to establish the point that they are from God. Only then can that point be used on behalf of their acceptability.