What is Idealism?:
Idealism is the category of philosophical systems that claim reality is dependent upon the mind rather than independent of the mind. Extreme versions of Idealism deny that any 'world' exists outside of our minds. Narrower versions of Idealism claim that our understanding of reality reflects the workings of our mind first and foremost - that the properties of objects have no standing independent of the minds perceiving them.
Important Books on Idealism:
The World and the Individual, by Josiah Royce
Principles of Human Knowledge, by George Berkeley
Phenomenology of Spirit, by G.W.F. Hegel
Important Philosophers of Idealism:
Gottfried Wilhelm Leibniz
Georg Wilhelm Friedrich Hegel
What is the "Mind" in Idealism?:
The nature and identity of the "mind" upon which reality is dependent is one issue that has divided idealists of various sorts. Some argue that there is some objective mind outside of nature, some argue that it is simply the common power of reason or rationality, some argue that it is the collective mental faculties of society, and some focus simply on the minds of individual human beings.
According to Platonic Idealism, there exists a perfect realm of Form and Ideas and our world merely contains shadows of that realm.
According to Subjective Idealism, only ideas can be known or have any reality (it is also known as solipsism).
According to Transcendental Idealism, developed by Kant, this theory argues that all knowledge originates in perceived phenomena which have been organized by categories.
According to Absolute Idealism, all objects are identical with some idea and the ideal knowledge is itself the system of ideas. It is also known as Objective Idealism and is the sort of idealism promoted by Hegel. Unlike the other forms of idealism, this is monistic - there is only one mind in which reality is created.