Idealism Overview

Idealism is a philosophical viewpoint that asserts that reality - or reality as we can know it - is fundamentally mental or spiritually constructed.
Seen as a direct contrast to materialism which holds that the primary substance of reality is material or physical.
It focuses on ideas and cognition, often discounting the importance of physical matter.
Most versions of idealism, especially subjective idealism, assert the unity of mind and world; they maintain that the real objects of our knowledge are ideas or representations in the mind.

Key Figures

George Berkeley was an influential idealist philosopher who proposed the theory ‘esse est percipi’ meaning ‘to be is to be perceived’.
Other significant idealists include Plato with his Theory of Forms, and Immanuel Kant with his transcendental idealism.
Friedrich Hegel introduced the concept of absolute idealism which posits that the reality of the mental world progresses according to dialectic patterns.

Types of Idealism

Subjective Idealism: Believes existence of objects is dependent on being perceived by minds. In this perspective, if there are no minds around, objects cease to exist.
Objective Idealism: Asserts that the existence of objects remains irrespective of whether minds exist to perceive them. They maintain that objects have an inherent reality, a reality fashioned by supernal or divine cognition.
Transcendental Idealism: Introduced by Immanuel Kant. He opines that our understanding of reality is not a direct reflection of things as they truly are but is a product of our mind’s structure and linguistic predicates.

Criticisms of Idealism

Accused of being anti-scientific as it goes against the fundamental premise of the empirical sciences which assume a world existing independent of the mind.
Critics often challenge its ability to explain shared experiences - if reality is mind-dependent, then it would be impossible for two individuals to have identical perceptions of the same object. Critics also question how multiple minds perceive the same reality.
Criticized for being counterintuitive and failing to align with common sense views about the world.
Materialists in particular reject the fundamental premise of idealism that reality is mind-dependent.

Form a thorough understanding of idealism, its types, key proponents, and criticisms to strengthen your epistemological understanding. Grasping this profound philosophy will not only help you appreciate its influence on various knowledge sectors but also develop your analytical skills on a broad philosophical spectrum.

Remember that the key to understanding and revising philosophy is to think critically about these perspectives, asking how and why they could answer the key epistemological questions and criticisms against them. This will help you draw comparisons and conclusions, which is a critical skill in epistemology.