The essence of Berkeley's philosophy was his assertion that matter doesn't exist.
He held that all objects perceived outside ourselves are simply ideas that exist only in the mind. He radically claimed that 'esse ist percipi' - meaning 'to be is to be perceived'.
This philosophy, and others like it, which take the view that the external world is somehow produced by the mind, is known as idealism.
As well as being considered an empiricist, Berkeley is also widely known as the father of philosophical idealism. He maintained that God, in whose mind all things exist at all times, implants in us in an orderly manner, all ideas. Therefore reality, or knowledge of the world, consists of the rational communication of ideas between God's eternal mind and our infinite minds.
Unlike John Locke, who also argued that ideas rather that external objects themselves are perceived, Berkeley stated that there is no distinction between primary (mind independent or existing in objects) and secondary (mind dependent or existing in us) qualities. He maintained there is no way of knowing whether our ideas of things are correct representations of that which they are supposed to represent. We have no reason to suppose they are caused by external objects, so therefore there can be no material substance. Hence:
"When we do our utmost to conceive the existence of external bodies, we are all the while only contemplating our own ideas."
For Berkeley, there is likewise a definite distinction between ideas and minds. All the objects of human knowledge are either ideas immediately experienced through sensation (taste, feel, seeing), ideas we hold from thinking about our emotions, or ideas formed from memory and imagination. The mind is not one of our ideas "but a thing entirely distinct from them, wherein they exist…whereby they are perceived; for the existence of an idea consists in being perceived."
When Berkeley received objections and criticisms regarding the role of science and various physical scientific findings, he argued that the proclamations of science were useful theories rather than factual accounts.
In one of his best-known works, Principles of Human Knowledge, Berkeley clearly and plainly presents the argument against materialism, extols the tenets of idealism, and puts forth a proof of the existence of God.
Berkeley is also know for his critique of abstraction (that which does not exist, but is merely man’s epistemological method of perceiving that which exists—and that which exists is concrete) which is an important premise for his argument on non-materialism.
A famous verse that enshrines Berkeley's doctrine was written by Ronald Knox and is as follows:
There was a young man who
Must think it exceedingly odd
If he finds that this tree
Continues to be
When there's no one about in the Quad.'
Your astonishment's odd;
I am always about in the Quad.
And that's why the tree
Will continue to be
Since observed by,