Most 18th-century thinkers gave up the quest for metaphysical knowledge after imbibing Bayle’s arguments. The Irish bishop George Berkeley, an empiricist and idealist, fought skeptical doubts by identifying appearance and reality and offering a spiritualistic metaphysics. He was immediately seen as just another skeptic, however, since he effectively denied the existence of a world beyond experience.
Bayle’s chief 18th-century successor was David Hume. Combining empirical and skeptical arguments, Hume asserted that neither inductive nor deductive evidence can establish the truth of any matter of fact. Knowledge can consist of intuitively obvious matters or demonstrable relations of ideas but not of anything beyond experience; the mind can discover no necessary connections within experience nor any root causes of experience. Beliefs about the world are based not upon reason or evidence, nor even upon appeal to the uniformity of nature, but only on habit and custom (see induction, problem of). Beliefs cannot be justified. Belief that there is an external world, a self, and a God is common, but there is no adequate evidence for it; and although it is natural to hold these convictions, they are inconsistent and epistemologically dubious. “Philosophy would render us entirely Pyrrhonian,” Hume declared, “were not Nature too strong for it.” The beliefs that a person is forced to hold enable him to describe the world scientifically, but when he tries to justify them he is led to complete skepticism. Before he goes mad with doubts, however, Nature brings him back to common sense, to unjustifiable beliefs. Hume’s fideism was a natural rather than a religious one; it is only animal faith that provides relief from complete doubt. The religious context of skepticism from Montaigne to Bayle had thus been removed, and humanity was left with only its natural beliefs, which might be meaningless or valueless.
The French Enlightenment philosophers, the philosophes, built upon their skeptical readings of Locke and Bayle and on their interpretation of Berkeley as a radical skeptic. While they produced vast amounts of new knowledge, they also placed alongside this a skepticism about whether one could ever establish knowledge of an external reality. Perhaps the most skeptical of the philosophes was the great French mathematician Condorcet (1743–94), who held that mathematics, physics, and moral philosophies were all merely probable. He also raised the possibility that the mental faculties by which people judge their knowledge might change over time, and hence that what is judged true today might not be judged true tomorrow.
The central themes in Hume’s skeptical analysis—the basis of induction and causality, knowledge of the external world and the self, proofs of the existence of God—became the key issues of later philosophy. Hume’s contemporary Thomas Reid hoped to rebut Hume’s skepticism by exposing it as the logical conclusion of the basic assumptions of modern philosophy from Descartes onward. Such disastrous assumptions, he urged, should be abandoned for commonsensical principles that have to be believed. As Hume and Kant saw, however, Reid had not answered Hume’s skepticism but had only sidestepped it by appealing to common sense. This provided neither a theoretical basis for belief nor a refutation of skeptical arguments.
Kant saw that Hume had posed a most fundamental challenge to all human knowledge claims. To answer him, it had to be shown not “that” knowledge is possible but “how” knowledge is possible. Kant combined a skepticism toward metaphysical knowledge with the contention that certain universal and necessary conditions are involved in having experience and describing it. In terms of these conditions it is possible to have genuine knowledge about the forms of all possible experience—space and time—and about the categories in which all experience is described. Any effort to apply these categories beyond possible experience, however, leads to contradictions and skepticism. Thus it is not possible to know about “things-in-themselves” or about the ultimate causes of experience.
Although Kant thought that he had answered the challenge of skepticism, some of his contemporaries saw his philosophy as commencing a new skeptical era. G.E. Schulze (or Schulze-Aenesidemus), a notable critic of Kantianism, insisted that, on Kant’s theory, no one could know any objective truths about anything; he could only know the subjective necessity of his own views. The Jewish critic Salomon Maimon contended that, though there are such things as a priori concepts (concepts that can be known independently of experience), their application to experience is always problematical, and whether they apply can be determined only through experience itself. Hence, the possibility of knowledge can never be established with certainty. Assured truth on the basis of concepts is possible only of human creations, like mathematical ideas, and it is questionable whether these have any objective truth. The thesis that human creativity is the basis of truth, however, was soon developed by Johann Gottlieb Fichte, a leading German idealist, as a new way of transcending skepticism.
Another skeptical critic of Kant, Johann Georg Hamann, saw in Hume’s and Kant’s work a new basis for fideism. If knowledge of reality cannot be gained by rational means, then one must turn to faith. Based on Hume’s efforts, Hamann advanced an antirational skepticism in an unsuccessful effort to convince Kant to become a fideistic Christian. Hamann’s kind of fideism was also developed in France by Catholic opponents of the French Revolution such as Joseph de Maistre and Félicité Lamennais.