George Berkeley's Treatise is largely a response to the intellectual storm stirred up by Locke and Descartes, even quoting Locke on several occasions within the Treatise.
Berkeley would be described in current philosophical terms as an idealist. The main purpose of his treatise is to show that we are all spirit and reality does not exist outside our perception of it. The world is sustained by God and the perceptions of men. No objects have an "essence" or even a "substance", they only exist as qualities that we perceive in them.
The world according to Berkeley is very "real" in the sense that it follows natural laws set by God and may be interacted with by other lesser spirits (men) in a consistent manner, but he rejects entirely the notion of matter.
This form of idealism solves many of the tricky philosophical problems with the existence of matter, its interaction with space and time, and the definition of infinitesimals, but I believe it opens up a different (and nastier) can of worms. For example:
"33 ... The ideas imprinted on the Senses by the Author of nature are called REAL THINGS; and those excited in the imagination being less regular, vivid, and constant, are more properly termed IDEAS, or IMAGES OF THINGS, which they copy and represent. But then our sensations, be they never so vivid and distinct, are nevertheless IDEAS, that is, they exist in the mind, or are perceived by it, as truly as the ideas of its own framing. The ideas of Sense are allowed to have more reality in them, that is, to be more STRONG, ORDERLY, and COHERENT than the creatures of the mind; but this is no argument that they exist without the mind. They are also LESS DEPENDENT ON THE SPIRIT, or thinking substance which perceives them, in that the are excited by the will of another and more powerful spirit; yet still they are IDEAS, and certainly no IDEA, whether faint or strong, can exist otherwise than in a mind perceiving it."
"36 ... There are spiritual substances, minds, or human souls, which will or excite ideas in themselves at pleasures; but these are faint, weak, and unsteady in respect of others they perceive by sense -- which, being impressed upon them according to certain rules or laws of nature, speak themselves the effects of a mind more powerful and wise than human spirits. These latter are said to have more REALITY in them than the former: by which is meant that they are more affecting, orderly, and distinct, and that they are not fictions of the mind perceiving them. And in this sense the sun that I see by day is the real sun, and that which I imagine by night is the idea of the former."
If the only difference between reality and imagination is the magnitude of "affection, order, and distinction", then it makes sense based on Berkeley's premises that our ideas are indeed weak reflections of reality. But this is a one-way sieve. It is clearly evident that our momentary experiences are distinct from our mental recall or reconstruction of them, but why can't we simply imagine an experience with enough order and distinction to become reality? Let's suppose that we take a hip new psychedelic and hallucinate a scene vividly with just as much order and distinction as our sober perception of reality. Does it then become "real"? Can we actually create the world around us by willing the alterations of our own perceptions so long as they are orderly and distinct? This is the weak link of Berkeley's treatise, and unfortunately also the foundation on which he builds the rest of his arguments.
Worth the read if you are interested in understanding one of the influential figures of modern idealism.