In philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...

, idealism is the family of views which assert that reality, or reality as we can know it, is fundamentally mental, mentally constructed, or otherwise immaterial. Epistemologically, idealism manifests as a skepticism
Skepticism has many definitions, but generally refers to any questioning attitude towards knowledge, facts, or opinions/beliefs stated as facts, or doubt regarding claims that are taken for granted elsewhere...

 about the possibility of knowing any mind-independent thing. As an ontological
Ontology is the philosophical study of the nature of being, existence or reality as such, as well as the basic categories of being and their relations...

 doctrine, idealism goes further, asserting that all entities are composed of mind or spirit.
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