Works
1. An Essay Towards a New Theory of Vision 1709
2. A Treatise Concerning The Principles of Human Knowledge 1710
3. Three Dialogues Between Hylas and Philonous 1713

Philosophy
1. Locke maintained that our knowledge does not reach beyond our "ideas."
Therefore,
Truth is an agreement between ideas, and not between an idea and its object.
then, Locke recognized and talked about the qualities of things (=primary qualities)
the certainty of sensory knowledge about
the external world.
Locke is not consistent in his thinking, e.g., on the one hand, we are not able to know
the object or material substance itself and yet
contended as if he knew the existence of the external world and of its objects.

Berkeley advanced a step further and radicalized Locke's empiricism by being more consistent in taking an idealistic position.

Berkeley denied the distinction of the primary and the secondary qualities,
denied the existence of the primary qualities independent of mind and
considered the primary qualities as additional relations to the secondary qualities.

Berkeley denied the existence of material substance and
held that substance must be incorporeal, thus only spirit deserves the name of substance.

Berkeley argued that the universal idea of substance is senseless
and
held that the particular idea of substance is no other than a combination of many ideas
which are also particular.

Berkeley contended that what we recognize is solely ideas.

To Berkeley, therefore,
Being or what really exists is either ideas (those which are perceived)
or the spirit or mind (that which actively perceives)

2. Berkeley radicalized Locke's' nominalism a step further and
denied the existence of the universal ideas produced by understanding
in stead
Berkeley advanced the Representational Theory to explain the universals.
According to Berkeley,
Try to imagine in your mind the number 3 as such or the triangle as such which is not particular. We discover that it is impossible to do so. What we can represent in our mind is only a particular idea and let this particular idea represent all the possible ideas e.g. of the number 2 or of the triangle. This psychological fact that we are not able imagine a universal idea and what we call the universal in the actual situation is to let a particular idea represent the rest. This is a purely psychological explanation, which Berkeley confused with the epistemological understanding of the universal.
Berkeley's Representational Theory of the Universal.

According to Berkeley,
Ideas are "sensible things", i.e., those which are perceived by our senses

Berkeley's radical empiricism resulted in the denial of the universal
and
came to maintain that the material world be obsolete.
(to a great extent, this has something to do with his linguistic analysis of the meaning of such terms as "existence," "primary qualities," "objects," in relation to the so-called "external world."
The ambiguous meaning of "being perceived" and "being known."
To Berkeley,
an "idea"= an "object" (Is this correct? No!)
The linguistic analysis is to be evidenced, according to Berkeley by concrete psychological experience (a particular idea or image).

To Berkeley,
A) The Notion of the Material Substance is obsolete in explaining the phenomena which presumably presupposes the material substance.
B) The Notion of the Material Substance is erroneous
for
i) "the object exists without being perceived." is a contradictory statement, once we comprehend the meanings of such terms as "object", "existence."
ii) it is also contradictory to maintain that an idea or a sensation is a copy of something which itself is neither an idea nor a sensation: for being is either "active" (as perceiving) or is, in relation to this being, "passive" (as being perceived) and
this "something" cannot be here "being" in either of these two senses.
iii) The sensory qualities are subjective conditions (e.g. sweet or white).
the so-called "material substance" is the aggregate of these sensory qualities (e.g. sugar)

Berkeley maintained that
the Material Substance does not exist, while

the Spiritual Substance is the only substance that exists.

"Esse est percipi."
This is the criterion for something (other than the Spiritual Substance) to be real
Let us consider that we are debating whether or not the ghost exists.
What is our criterion for the existence of anything including the ghost?
We normally say,
"Oh, yes. Once we see the ghost, we believe that the ghost exists."
In other words, we takes the perception of something for its necessary condition for the existence.
If anything exists, then it must be perceived.
Berkeley took this being perceived rather the sufficient condition for its existence (=being).
(however, it is omitted in this discussion that the spiritual substance which perceives also exists. In other words, If anything is perceiving, then it exists.

As to the Material Substance,
An Idea does not refer to anything else but another idea.
The false belief in the external, material substance is resulted from the fact that some ideas are indeed not arbitrary, namely
sensations (i.e, ideas of senses) are more strong, vivid, clear and constant, further in accordance with regular unity and the order. They are felt compulsory in perceiving.

We believe, according to Berkeley, that
If they are not produced arbitrarily by us, there must be some "External Cause"
that Wills and Thinks.
For unless it can will, it cannot actively influence (cause) us to have these ideas.
Our sensations are regular and of multiplicity,
Therefore their Cause must have Infinite Power and Knowledge.
This External Cause is God.
Imaginations are caused by us
Sensations are Caused by God

the totality of the ideas caused by God = Nature
the continuous, constant order of nature = the Laws of Nature

Through those ideas of senses caused by God = copies of Divine Eternal Ideas.

That the "material things" do not exist apart from their "perceptions" does not imply that
they depend upon individual spirits.

Freedom of Will
Berkeley did not comprehend the freedom of will and yet he believed that we should recognize it.

According to Berkeley,
Nature is construed by Berkeley rather teleologically than mechanistically.
For the cause of all the (efficient) causes cannot be found in causality, but must be searched in God as the ultimate Cause or Explanation.

Berkeley's Idealism:
Being is either 1) perceiving or
2) being perceived. This latter was exaggerated.

We may find common characteristics among Berkeley, Leibniz and Fichte:
1) Only the Spirit is Active
Thus only active is "existing." (=real)
2) All those which are not active, i.e.,
can exist by being perceived (being acted upon by the cognitive subject)
(in case of Berkeley, each (sensory) impression of idea is impressed to each finite Spirit by the Infinite Spirit (=God)).
In Case of Leibniz, by the pre-established harmony, at the, everything was "implanted" in the monad.
In case of Fichte, each "objective" idea is a product of the absolute Ego unconsciously active in each finite individual ego.
There are as many worlds as monades or spirits for Leibniz and Berkeley, while there is only one world for Fichte. For the absolute Ego is not transcendent, but immanent in each finite ego (just as Kant's transcendental subjectivity is "immanent" in each empirical subject).