The common-sense philosophers of the Scottish school—including Thomas Reid, Dugald Stewart, James Beattie, George Campbell, and James Oswald—argued against George Berkeley and David Hume that ordinary human perception and moral judgment need not be defended against skeptical inquiry but ought to be taken as self-evident. As Campbell put it, "to maintain propositions the reverse of the primary truths of common sense, doth not imply a contradiction, it only implies insanity" (The Philosophy of Rhetoric, 1776). The common-sense school was criticized by Immanuel Kant for its "appeal to the opinion of the multitude" (Prolegomena, 1783); Joseph Priestley, for example, wrote in his "examination" of Reid, Beattie, and Oswald (1774) that common sense is for persons of "middling" capacities. Nevertheless, their ideas enjoyed immense influence, not only in Great Britain but also in Germany and elsewhere. The idea that ordinary language can express principles of common sense, emphasized in the works of the Scottish school and also present in the work of the Italian jurist Giambattista Vico, led to interest in common sense on the part of later philosophers, including Henry Sidgwick, G. E. Moore, Hans-Georg Gadamer, J. L. Austin, and Hannah Arendt.
While most members of the Scottish school are read today primarily for historical interest, the work of the Presbyterian minister and professor of moral philosophy, Thomas Reid, has undergone a revival in recent years. Reid began his career at Marischal College in Aberdeen, where he founded the Aberdeen Philosophical Society, or "Wise Club." The group concentrated much of its energy on the work of fellow Scot David Hume. While the poet, zealous Christian, and anti-Humean polemicist, James Beattie, joined the society some nine years after its founding, most of its members were more concerned with establishing an empirical foundation for British learning than with combating potential heresy. Hume called Beattie's Essay on the Nature and Immutability of Truth (1770) "a horrible large lie in octavo"; though Hume disagreed with Reid's criticism of his work and once suggested in a letter to a mutual friend that pastors ought to stick to "worrying" each other and leave the philosophers to their arguments, he generally treated Reid with respect.
Hume's work was of great interest to the Aberdeen philosophers—and to members of the Scottish common-sense school in general—because it brings Cartesian skepticism to conclusions that, for them, are so contrary to ordinary human experience that they demonstrate the futility of all such philosophizing. In his Inquiry (1764), Reid writes that "since we cannot get rid of the vulgar notion and belief of an external world, [we ought] to reconcile our reason to it as well we can." Half a century earlier, Anthony Ashley Cooper, third earl of Shaftesbury, had argued in a commonsensical vein that it would be easier to imagine half of mankind mad than to deny the conclusions of "natural knowledge, fundamental reason, and common sense" ("Sensus Communis," 1710). For Shaftesbury, human beings have a natural faculty of moral sense.
One difficulty with tracing the influence of the idea of common sense is the variety of meanings attached to it even by thinkers in close geographical and historical proximity. Reid's commonsense access to moral judgment is much more like reasoned appeal to a self-evident principle than Shaftesbury's combination of feeling and thinking, and Francis Hutcheson's use of the term "moral sense" implies still less reason and still more immediate sensation than Shaftesbury's. Reid's realism is starkly opposed to Berkeleyan idealism, yet both authors appeal directly to the authority of common sense.
These difficulties aside, however, it is possible to specify several important views that set the Scottish common-sense school apart. First, common-sense philosophy criticized both moral and epistemological skepticism from the point of view of ordinary reason. This argument sometimes took the form of ad hominem attack on Hume: "Even the author of the Treatise of human nature, though he saw no reason for this belief [in hardness of bodies in nature], but many against it, could hardly conquer it in his speculative and solitary moments; at other times he fairly yielded to it, and confesses that he found it necessary to do so" (Reid, Inquiry, 1764).
Second, philosophy takes its starting point from the self-evident principles of common sense. For Oswald, these self-evident principles included many Christian religious doctrines, and indeed a good bit of the common-sense school's subsequent popularity lay in its claim to defend the religious views of ordinary British citizens from the perceived incursions of system-building philosophers. Reid's position on religion and self-evidence is more complex and interesting; for example, he argues that it makes no sense to deny the validity of perception while accepting that of reason because both faculties "come out of the same shop."
Third, not only does common sense furnish pragmatic certainty about the evidence of our senses and the existence of our selves, but it also provides natural insight into moral questions. Along with commonsense views of right and wrong comes the conviction of one's own human agency. Reid developed this idea into what contemporary philosophers would call an incompatibilist defense of freedom: for Reid, free human agents begin causal chains of events independent of natural causes.
The German reception of the Scottish common-sense philosophers is frequently described by commentators as a "misreception." Generally, while German readers were enthusiastic about the contributions of the Scots, they tended to empty the idea of "common sense"of the moral and social elements present in Reid's and others' work. The "popular philosophy" movement of the mid-eighteenth century in Germany shared the commonsense zeal of the Scottish school, and members of both groups criticized the absurdities of academic philosophizing. As Christian Garve put it (writing, in 1798, his observations on the most common principles of ethics), common sense is not "common because it is contemptible, but because it is, or should be, the common property of all human beings" Unlike the popular philosophers, Kant was highly critical of the Scottish common-sense school and its reading of Hume in his Prolegomena (1783); however, he took the concept of common sense itself seriously in his Critique of Judgment (1790). There Kant distinguished between "sensus communis," considered as an idea of a comparative sense, and "common human understanding," which includes qualities exhibited by all normal human reason.
Of course Kant, unlike the Scottish school, denies that either sensus communis or common sense frees us from the need to justify our sensations and beliefs. However, Kant did provide some intriguing suggestions about both concepts' functions; each, in a different way, serves to replace unavailable certainty in judgment with an approximation of collective judgment. This aspect of Kant's treatment of common sense inspired Hannah Arendt's interesting independent reflections on the topic, which include the suggestion that when it comes to choosing between moral approbation and disapprobation, the "criterion … is communicability, and the standard of deciding about it is common sense."