Esse est percipi
To be is to be perceived.
Alice C. Linsley
George Berkeley (1685-1753) was an Irish philosopher who became the Bishop of Cloyne. He lived for a time in the American colonies where he promoted higher education. He deeded his library and his farm in Rhode Island to Yale University. One of the colleges at Yale is named after him, as is the city of Berkeley in California.
Berkeley saw a flaw in the thought of Rene Descartes who believed that objects have existence independent of our perception because they take up space, that is, they are "extended." Berkeley argued that there is no "outside" world, only the world of the Mind. Speaking of mind, many people believed that poor George had lost his! The importance of his thought was appreciated only after his death.
In his writings, Berkeley shows himself to be an consistent thinker. He argues that we cannot logically insist that objects have existence independent of human perception, that they take up space in the material sense. This leads to the conclusion that all that exist is Mind and the content of Mind: thoughts, ideas, and perceptions. Berkeley is sometimes called an "idealist" because he believed that all things that exist are ideas. He is also called an "immaterialist" because he denied that material objects exist. Berkeley applied this reasoning to God, saying that everything has existence in the Mind of God (panentheism).
One of Berkeley's contemporaries was Samuel Johnson, who hearing about Berkeley's theory, kicked a stone and declared, "I refute it thus." For Johnson feeling that hard stone against his toe was sufficient evidence that the stone had material substance. However, Johnson missed Berkeley's point. Feeling the stone against your foot did not prove that the stone had extension. It only proved that the mind has an idea of a hard stone.
Berkeley develops his immaterialism in his Three Dialogues Between Hylas and Philonous (1713). The word hylas comes from the Greek word for matter, and philonous means love of Mind. In these dialogues, Berkeley shows that sensible qualities are not inherent in matter. Rather, they are ascribed and understood by the mind. Color, sound, temperature and shape are relative qualities entirely dependent on a mind. Without a mind, there is no perception of matter at all.
Berkeley sought to refute the claims of his contemporary John Locke about the nature of human perception. According to Locke, a thing's primary qualities, such as its extension, shape, motion, solidity, and number, exist apart from being perceived. Berkeley, on the other hand, insisted that primary qualities are ideas that exist in a perceiver's mind. These ideas cannot exist in an unperceiving substance.
Berkeley's method in his own words from A Treatise Concerning the Principles of Human Knowledge (1710)
Philosophy being nothing else but the study of wisdom and truth, it may with reason be expected that those who have spent most time and pains in it should enjoy a greater calm and serenity of mind, a greater clearness and evidence of knowledge, and be less disturbed with doubts and difficulties than other men. Yet so it is, we see the illiterate bulk of mankind that walk the high-road of plain common sense, and are governed by the dictates of nature, for the most part easy and undisturbed. To them nothing that is familiar appears unaccountable or difficult to comprehend. They complain not of any want of evidence in their senses, and are out of all danger of becoming Sceptics. But no sooner do we depart from sense and instinct to follow the light of a superior principle, to reason, meditate, and reflect on the nature of things, but a thousand scruples spring up in our minds concerning those things which before we seemed fully to comprehend. Prejudices and errors of sense do from all parts discover themselves to our view; and, endeavouring to correct these by reason, we are insensibly drawn into uncouth paradoxes, difficulties, and inconsistencies, which multiply and grow upon us as we advance in speculation, till at length, having wandered through many intricate mazes, we find ourselves just where we were, or, which is worse, sit down in a forlorn Scepticism...
My purpose therefore is, to try if I can discover what those principles are, which have introduced all that doubtfulness and uncertainty, those absurdities and contradictions into the several sects of philosophy; insomuch that the wisest men have thought our ignorance incurable, conceiving it to arise from the natural dullness and limitation of our faculties. ...
Nothing seems of more importance, towards erecting a firm system of sound and real knowledge, which may be proof against the assaults of scepticism, than to lay the beginning in a distinct explication of what is meant by thing, reality, existence: for in vain shall we dispute concerning the real existence of things, or pretend to any knowledge thereof, so long as we have not fixed the meaning of those words...
.. we are under an invincible blindness as to the true and real nature of things... Hence a great number of dark and ambiguous terms presumed to stand for abstract notions, have been introduced into metaphysics and morality, and from these have grown infinite distractions and disputes amongst the learned.
Related reading: George Berkeley: Idealist and Consistent Empiricist; A Treatise Concerning the Principles of Human Knowledge by George Berkeley