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In order to understand how a conservative such as Bishop Berkeley could have been driven to posit a system as radical as idealism (and to have the boldness to call this view "common sense") it is important to get a handle on two other philosophical systems of the era: the groundbreaking rationalism of René Descartes and the level-headed empiricism of John Locke.
Descartes, born in 1569, was not the first scientist to develop a mechanistic, mathematical science, though he was influential in its development, and perhaps most ambitious in his scope. He was, however, the first to give a thorough and comprehensive philosophical response to the demands raised by this new way of viewing the world. His writings initiated a dramatic revision of philosophical method and concerns. In order to clear the way for a new scientific outlook, Descartes had to dramatically simplify the metaphysical picture of the world. Where the Scholastics (the reigning leaders of the intellectual world at the time) had posited numerous types of substances, each with their own essence, and each requiring their own type of explanation in terms of earth, air, fire, and water, Descartes argued that there were only two types of substance in the world. There was mental substance, whose essence was thinking, and there was physical substance, whose essence was extension. Since the entire observable world thus reduced to a single sort of substance (i.e. physical substance or body), all natural phenomena could be explained by relying on just a small number of principles, based entirely on the property of extension. Physics conveniently collapsed into geometry, the study of extended body.
Given his mechanistic picture of the world, on which all explanation could be given in terms of the extension of physical substance, Descartes also needed a new epistemology, or theory of cognition, to complement his new physics and metaphysics. Scholastic philosophers, following Aristotle, believed that all human knowledge comes through the senses. That is to say, they were empiricists. However, their empiricism was of a very naïve form; they believed that our senses are incapable of systematically deceiving us about the kinds of things that are in the world. If the senses tell us that there are colors, then there are colors. If the senses tell us that there are enduring objects, such as tables and chairs, then there are enduring objects. The trustworthiness of the senses was built into the conception of how perception operated: the one perceiving, on this view, took on the form of the thing perceived, became, in a very obscure sense, like the object of perception. Yet on Descartes' metaphysical picture of the world, there was no such thing as color, sound, odor, taste, heat. There was only extension and the properties that arose from it, such as size, shape, and motion. In order to defend his physics and metaphysics, therefore, Descartes was forced to come up with a new understanding of where human knowledge comes from. Knowledge could not come from our senses, because our senses tell us that we live in a colorful, loud, odorous, tasty, hot, cold world.
In order to rid knowledge of sensory influence, Descartes' freed the intellect from the senses altogether. Where the Scholastics had claimed that nothing got into the intellect except through the senses, on Descartes' theory of cognition, certain concepts are present in the intellect at birth. According to Descartes, human beings are born with certain innate concepts, concepts such as "God," "extension," "triangle," and "something cannot come from nothing." Using these innate concepts, and our faculty of reason, we can trace chains of logical connections and unravel all the possible knowledge in the world.
Like Descartes, John Locke, was a proponent of the new science. He too believed that the natural world was explicable exclusively in terms of shape, size, and motion of matter, though the particulars of the view he ascribed to were somewhat different from the Cartesian picture. (Whereas Descartes believed that all matter was continuous, Locke ascribed to the chemist Robert Boyle's Corpuscular Hypothesis according to which the natural world is composed of indivisible bits of matter called corpuscles.) He had to admit, therefore, that Descartes was right about at least one thing: the senses do systematically deceive us. Locke, however, could not bring himself to accept Descartes' epistemology. Like the Scholastics, Locke believed firmly that nothing came into the mind without first coming through the senses. His work on epistemology and metaphysics (which can be found in the Essay Concerning Human Understanding, published in 1671, twenty years after Descartes' death), therefore, is an attempt to reconcile his empiricism with his commitment to the new science. His aim was to defend an empiricist model of the mind, while clearing the way for new ideas about the nature of reality.
The mixture of a Cartesian metaphysics and an empiricist epistemology, however, led Locke into many difficulties. According to the Cartesian metaphysics, the world as we experience it through our senses (that is, as colored, tasty, odorous, full of sound) is different from the way the world really is (that is, filled only with colorless, tasteless, scentless, soundless matter); but according to an empiricist epistemology, our only access to the world is through our senses. Taking this blend of ideas to its logical conclusion, then, Locke's philosophy seems to lead straight into skepticism: we cannot know what the world is really like; we cannot know the true nature of things. Only adding to the force of this skeptical conclusion is Locke's theory of perception, also taken over from Descartes. According to this theory, we do not have immediate access to the world, but, rather, we see the world through an intermediate layer of ideas, often referred to as the "veil of perception". In other words, objects in the world cause ideas to spring up in our mind, and it is these ideas, and not the objects themselves, that we see when we look out around us. But if we have no immediate access to the world, one might reasonably ask, how on earth do we know if our ideas resemble what is really out there? Descartes was able to get around this worry, by claiming that we can know about the world through our purely intellectual, innate ideas, but Locke, as an empiricist, could not use this escape hatch. His philosophical system, then, not only leads to the worry, "can we know about the true nature of things," it also leads to the worry that, for all we know, the world as it is, is nothing like the world as we experience it. The entire world could actually be, say, one giant, undifferentiated ball of jello (with no objects, bodies, etc. in the mix), and we would be none the wiser.
Locke himself strenuously resisted any skeptical conclusions. In fact, he did not even take the threat of skepticism seriously. Berkeley, however, did take this threat seriously, and he viewed the Lockean mixture of Cartesian metaphysics and empiricist epistemology with a deep suspicion. As a committed empiricist himself, Berkeley needed to find a way to avoid the skeptical conclusions that Locke's philosophy seemed to lead to. His solution was to scrap half of the Cartesian metaphysics, eliminating matter and keeping only mind. By claiming that all there is in the world is ideas and the minds that perceive them, Berkeley was able to avoid the worries that crept up for Locke. The world, on this view, must really be colored, tasty, smelly etc. because the world just is our ideas. Therefore, we can also be certain that we know the true nature of things. In addition, there is no veil of perception on Berkeley's picture, because the ideas to which we have immediate access are the real objects out in the world; there is nothing coming between us and really existing things. We never have to worry, then, that reality does not correspond to our perception of it; we know what exists out there, and what it is like.
Though Berkeley's solution might sound ludicrous (it certainly did to all of his contemporaries) it actually ended up becoming widely influential. In the 19th century, idealism became all the rage, beginning with Kant (who denied he was an idealist, but came close enough to be called one by most people since) and culminating in Hegel, Schelling, and the British Idealists like Greene, Bosanquet, Bradley, and Andrew Seth. Though these philosophers tended to denigrate Berkeley's importance, they owed their most fundamental ideas to his innovations, and even based their own arguments on ones that he himself had made two centuries earlier.
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