Idealism
Idealism is a form of monism which says that the single substance is Mind and that the material world is a construct of the mind rather than it having a separate existence.
Idealism comes in several versions.
Berkeley [1], Hegel, Kant plus some modern versions exist. According to this model, nothing material exists apart from what is in our minds. It is monism but is at the opposite pole from physicalism or materialism.
Hegel describes “Absolute Idealism” in his book "The Phenomenology of Spirit". According to Hegel, the “Absolute Spirit” is the ultimate reality that underlies all of existence, and individual minds are ultimately expressions of this universal consciousness.
Berkeley [1] describes “Subjective Idealism”. According to Berkeley, individual minds are distinct but ultimately depend on the perception of other minds and on a shared system of language and meaning.
A third approach taken by idealism is to argue that the existence of multiple minds is ultimately an illusion. This view is often associated with the Indian philosopher Advaita Vedanta, which holds that the ultimate reality is non-dual and that the apparent multiplicity of the world is ultimately a product of ignorance or misunderstanding.
There are similarities between Hegel's philosophy of absolute idealism and some Hindu and Buddhist views, particularly those of Advaita Vedanta and Mahayana Buddhism. However there are also important differences.
For example, Hegel emphasizes the role of self-consciousness and the rational structure of reality, while Advaita Vedanta emphasizes the role of intuition and direct realization of ultimate reality, and Mahayana Buddhism emphasizes the role of compassion and the liberation of all sentient beings from suffering.
Additionally, Hegel's philosophy is rooted in the Western philosophical tradition, while these Eastern views emerged from their respective cultural and religious contexts.
In one form of Idealism, solipsism, it is considered that there is only one Mind, one's own, and that everything and everyone else is a construct of it.
This view has the advantage that our experience would not be contradicted, things would seem to be exactly as they do. We would, effectively, be in a virtual reality scenario where what we perceive to be solid objects are actually constructs. It is hard to think of any experiment we could carry out which would enable us to tell the difference.
Other that Solipsism, a difficulty with this approach is that the external world is a shared one. We can interact with other minds using material as a go-between. Without postulating an elaborate shared virtual reality machine, such as the Matrix, this would not happen and doing so seems to add little to our understanding.
[1] Berkeley, G. (1948–1957). The Works of George Berkeley, Bishop of Cloyne. A.A. Luce and T.E. Jessop (eds.). London: Thomas Nelson and Sons. 9 vols.