Idealism

Idealism is any philosophy which argues that the only things knowable are consciousness or the contents of consciousness - not anything in the outside world, if such a place actually exists. Indeed, idealism often takes the form of arguing that the only real things are mental entities, not physical things.

Idealism comes in several flavors:

Subjective Idealism, only ideas can be known or have any reality (also known as solipsism).

Transcendental Idealism, developed by Kant, this theory argues that all knowledge originates in perceived phenomena which have been organized by categories.

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 monastic - there is only one mind in which reality is created.

Platonic Idealism, there exists a perfect realm of Form and Ideas and our world merely contains shadows of that realm.