A Treatise Concerning the Principles of Human Knowledge

Empiricism and rationalism dominated George Berkeley’s era. Empiricists emphasized the importance of experience, especially that gained through the senses, in the process of acquiring knowledge. From these direct observations, the human mind uses inductive reasoning to make larger generalizations and, thus, arrives at a more detailed knowledge of things.

By contrast, rationalists argued that the human mind can grasp truth through reasoning alone, without the mediation of the senses. Some rationalist thinkers believed that human beings possessed innate ideas or were born naturally knowing certain things. Empiricists, on the other hand, argued that the mind was a tabula rasa, or blank slate, and needed to be filled with sense experience to begin the process of knowledge. It has been argued that empiricism served as the foundation for the scientific method or studying the physical world through direct observation and experiment.

Empiricism became especially associated with British thinkers beginning in the late 17th century, while rationalist thought developed in France among philosophers such as René Descartes (1596-1650) and Nicolas Malebranche (1638-1715). The major British empiricist of Berkeley’s era was John Locke (1632-1704). In the Treatise, Berkeley quotes several passages from Locke (without attribution), including from the 1690 Essay Concerning Human Understanding, in which Locke describes the mind as a blank slate, which life fills with experience.

Berkeley posed what might be considered an extreme form of empiricism since he argues that the world must be experienced to exist. Berkeley argues that the laws of nature are signs in which God communicates ideas to human beings. Since everything we see is a result of sense experience and mental reflection, there is no reason to assume the existence of matter at all; rather, everything consists simply of ideas—things being perceived—and the minds that perceive them.

The third major philosophical movement relevant to the Treatise is skepticism, or doubt about the human ability to grasp truth—a perspective with origins in ancient Greece. After the period of Scholasticism in the Middle Ages, in which it was believed that reason and religious faith were in harmony, skepticism resurfaced during the Age of Reason. A number of 17th-century thinkers argued that nothing could be known for certain. Descartes established a form of “methodological skepticism” in which he rejected every idea that could be doubted, then rebuilt a firm ground for knowledge on the idea “I think, therefore I am.”

Berkeley wrote against skepticism. His mission to reestablish a firm basis for knowledge, which he does principally by equating existence with perception and by establishing God as the guarantor of truth as we perceive it. He sought to return to the unity of faith and reason that had formed the basis of medieval Christian thought.

Berkeley’s immaterialism was greeted with ridicule during his lifetime. Well-known is the reaction of the English writer Samuel Johnson; when told of Berkeley’s theory that all objects are merely ideal, he is reported to have kicked a stone and retorted “I refute it thus.”

Berkeley’s immaterialism, also known as subjective idealism, remains a major school of metaphysical thought on the nature of mind, experience, and perception.