Three Dialogues between Hylas and Philonous by George Berkeley

Despite the fact that Berkeley was at the forefront of one of the most outrageous trends in the history of philosophy (that is, idealism), he was actually a conservative; in fact, his radicalism grew out of his excessive conservatism. Faced with the freethinking 17th century scientists and writers who sought to overthrow traditional forms of religion, government, and conceptions of reality, Berkeley reacted by making a drastic philosophical move meant to prevent any further movement on these other fronts. By positing that the only things in the world are ideas and minds, Berkeley hoped to stem the threatening "freethinking" tide. As Berkeley himself succinctly puts it in the third dialogue, "That innovations in government and religion, are dangerous, and ought to be discountenanced, I freely own. But is there any like reason why they should be discouraged in philosophy?"

By Berkeley's time a new science was in full swing, pioneered by thinkers like Descartes and Galileo, and now in the hands of men like Sir Isaac Newton and Robert Boyle. This new science was mechanistic and mathematical in nature; it sought to explain all physical phenomena in terms of the motion of tiny particles of matter. The entire physical world, on this view, was made up of these particles, or corpuscles, with nothing else added. Only certain extremists, such as Thomas Hobbes, actually believed that this picture gave an exhaustive description of the entire universe. Most thinkers of this age, including both Descartes and Locke, believed that in addition to the physical objects in the world (which could be explained in these purely mechanistic terms) there were also spiritual entities, or souls, both human, angelic, and divine (i.e. God). But while the dualistic view of Descartes and Locke opened up a space for God, souls, and all the other necessary trappings of religion, Berkeley felt that the space it left open was both too small and too precarious.

God, in this mechanical world, became almost superfluous; He was appealed to only now and then to close up certain gaps in the otherwise self-sufficient theories. (Descartes, for instance, uses God to provide force in his physical system, and Locke uses God to bridge the explanatory gap between the world as we experience it and the world as it really is.) Giving God these minor causal roles was not sufficient in Berkeley's eyes; to him it was clear that God had to entirely ground any true description of physical reality. In addition, he recognized that it was only a matter of time before the mechanistic philosophers closed all their gaps and eliminated God from their systems altogether. Philosophers such as Thomas Hobbes and Baruch Spinoza were already taking these last steps toward a godless science, either chasing God from their picture entirely or giving God such an abstract, impersonal form as to make Him unrecognizable to any religious believer. Berkeley was not the only religious believer to view the creeping atheism with fear. "The church in danger", was actually a popular war cry at the time. However, he did battle these forces with unusual vigor, and also probably came up with the most original means by which to proceed: banishing matter from the world altogether. It was for these efforts that he was made Dean of Derry, and then, ultimately, Bishop of Cloyne.

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 gall 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 Rene Descartes and the level-headed empiricism of John Locke.

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.