George Berkeley
Contributions to philosophy

According to Berkeley there are only two kinds of things: spirits and ideas. Spirits are simple, active beings which produce and perceive ideas; ideas are passive beings which are produced and perceived.

The use of the concepts of "spirit" and "idea" is central in Berkeley's philosophy. As used by him, these concepts are difficult to translate into modern terminology. His concept of "spirit" is close to the concept of "conscious subject" or of "mind", and the concept of "idea" is close to the concept of "sensation" or "state of mind" or "conscious experience".

Thus Berkeley denied the existence of matter as a metaphysical substance, but did not deny the existence of physical objects such as apples or mountains ("I do not argue against the existence of any one thing that we can apprehend, either by sense or reflection. That the things I see with mine eyes and touch with my hands do exist, really exist, I make not the least question. The only thing whose existence we deny, is that which philosophers call matter or corporeal substance. And in doing of this, there is no damage done to the rest of mankind, who, I dare say, will never miss it.", Principles #35). This basic claim of Berkeley's thought, his "idealism", is sometimes and somewhat derisively called "immaterialism" or, occasionally, subjective idealism. In Principles #3, he wrote, using a combination of Latin and English, esse is percipi (to be is to be perceived), most often if slightly inaccurately attributed to Berkeley as the pure Latin phrase esse est percipi. The phrase appears associated with him in authoritative philosophical sources, e.g., "Berkeley holds that there are no such mind-independent things, that, in the famous phrase, esse est percipi (aut percipere) —to be is to be perceived (or to perceive)."

Hence, human knowledge is reduced to two elements: that of spirits and of ideas (Principles #86). In contrast to ideas, a spirit cannot be perceived. A person's spirit, which perceives ideas, is to be comprehended intuitively by inward feeling or reflection (Principles #89). For Berkeley, we have no direct 'idea' of spirits, albeit we have good reason to believe in the existence of other spirits, for their existence explains the purposeful regularities we find in experience ("It is plain that we cannot know the existence of other spirits otherwise than by their operations, or the ideas by them excited in us", Dialogues #145). This is the solution that Berkeley offers to the problem of other minds. Finally, the order and purposefulness of the whole of our experience of the world and especially of nature overwhelms us into believing in the existence of an extremely powerful and intelligent spirit that causes that order. According to Berkeley, reflection on the attributes of that external spirit leads us to identify it with God. Thus a material thing such as an apple consists of a collection of ideas (shape, colour, taste, physical properties, etc.) which are caused in the spirits of humans by the spirit of God.
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