The early modern philosophers of the seventeenth century accepted most of ancient thought about knowledge. The clear and distinct ideas that alone count as knowledge for René Descartes (1596–1650) are not notably different from the reasonable account that is Plato's criterion of epistēmē- knowledge. Even supposedly "empirical" philosophers like John Locke (1632–1704) assume that there is a strict and proper sense of knowledge that requires nothing less than rational certainty.
The break with tradition came from outside philosophical epistemology, in the new experimental natural philosophy of Galileo Galilei (1564–1642), Robert Boyle (1627–1691), Isaac Newton (1642–1727), and others. The first attempt to describe the experimental method was by Francis Bacon (1561–1626). The most influential account, however, is in Immanuel Kant's Critique of Pure Reason (1781). Kant famously explains how concepts are empty without a content they acquire from experience, and how sensations are chaotic noise without a priori concepts we bring to the process of understanding. In this account, empirical knowledge is a synthesis, a mental construction, combining what the senses offer with concepts that, in their broadest features, are a priori forms of human understanding.
Conceiving of knowledge as something put together in the service of understanding suggests that the control of experience may be a more important cognitive goal than the fidelity (or "correspondence") of a disinterested representation. This idea was explored in the nineteenth century by Arthur Schopenhauer (1788–1860) and Nietzsche. Further reflection on scientific experiments confirmed a similar view. What we learn from experiments is how to produce highly controlled effects, not how things are "in themselves," apart from the experimental intervention. This idea of knowledge as an external force of control was taken up by the Vienna positivists, including Ernst Mach (1838–1916), and the American Pragmatists—Charles Sanders Peirce (1838–1914), William James (1842–1910), and John Dewey (1859–1952), who reached conclusions not dissimilar to those of Nietzsche.
Sociology of knowledge.
Twentieth-century thinkers influenced by Auguste Comte (1798–1857) and Karl Marx (1818–1883) proposed a sociological theory of knowledge. The first premise of these theories is that no knowledge is entirely autonomous in structure or development from the group that produces it. How one looks at data, how one construes given facts, what one takes seriously, depend on social position. Karl Mannheim's seminal Ideologie und Utopie (1929; translated as Ideology and Utopia, 1936) argued that social circumstances determine both what we seek to know and the validity of knowledge attained. Later accounts abandon the idea of validity, rejecting the philosophical distinction between knowledge and doxa, an ideal subject matter for sociology.
For much of the twentieth century, philosophical discussion of knowledge was preoccupied with the problem of skepticism. Originally, skeptikos meant an inquirer, and later came to refer to followers of the Greek philosopher Pyrrho of Elis (c. 360–c. 272 B.C.E.). Their school flourished in the classical world between 100 B.C.E. and 200 C.E. These Skeptics taught the radical suspension of judgment, liberating the self of dogmatic convictions (and all convictions are dogmatic), as the way to mental tranquility (ataraxia). The point of skeptical arguments is to instill doubt about the most obvious matters, to show that belief is futile. Nothing can be proved because anything can be proved. There is no argument so convincing that an equally convincing argument for the opposite cannot be constructed. Mental peace lies in getting over the vanity of knowledge.
Skepticism fell into decline after Roman times. By the Middle Ages the school and its arguments were forgotten. This situation changed abruptly in the latter sixteenth century, when long-lost texts of ancient skepticism were republished. From then on skepticism played a role in early modern thought, especially in the work of Michel de Montaigne (1533–1592), Descartes, Pierre Bayle (1647–1706), and David Hume (1711–1776). Yet Kant and Georg Wilhelm Friedrich Hegel (1770–1831) both suppose they have overcome skepticism or shown it to involve a mistake, and for most of the nineteenth century skeptical problems were not much discussed in Euro-American philosophy.
In the twentieth century the so-called Analytic philosophers rediscovered skeptical problems as ideal for their methods of precise, rigorous, often logically formalized argumentation. Their problem is to prove the objectivity of knowledge, which usually means refuting the skeptic, who asks how you know that you are not dreaming, or are not a brain in a vat, or that the universe did not come into existence a minute earlier, complete with your faulty memories. The presumption is that unless we can prove that we can prove nothing, and unless something is proved there is no objective knowledge. Over a period of two thousand years, then, skepticism changed from being a way of life, as it was for Pyrrhonians, to a mood and method of self-knowledge in Montaigne and Descartes, to a technical problem for the most formidably technical work since high-Medieval scholasticism.