Idealism asserts that reality is primarily composed of mental or spiritual entities, rather than physical matter.

Materialism holds that matter is the sole fundamental substance from which all else arises.

Plato's theory of Forms posits that the physical world is an imperfect reflection of the eternal and unchanging realm of Forms.

Dialectical materialism, as expounded by Karl Marx and Friedrich Engels, asserts that the world is composed of matter in motion and that all phenomena are interconnected and interdependent.

René Descartes's 'cogito ergo sum' ('I think, therefore I am') is a foundational principle of modern philosophy, asserting the indubitability of one's own existence as a thinking being.

Critics of idealism argue that it is too abstract and divorced from the concrete realities of the physical world.

Critics of materialism argue that it cannot adequately explain consciousness and subjective experience, which seem to be irreducible to physical processes.

Immanuel Kant's distinction between the phenomenal world (as it appears to us) and the noumenal world (the true reality) is a central tenet of his transcendental idealism.

Subjective idealism asserts that reality is dependent on the individual mind, while objective idealism asserts that reality is independent of the individual mind.

Georg Wilhelm Friedrich Hegel's theory of objective idealism posits that the material world is a product of the collective human consciousness, or 'Absolute Spirit'.

Critics of dualism argue that it cannot adequately explain how the mind and the body interact, given their seemingly disparate natures.

Ludwig Feuerbach's materialism posits that the material world is the only reality and that consciousness is a product of the brain.

Critics of phenomenalism argue that it cannot adequately explain the existence of the external world, as it reduces everything to subjective sense-data.

Karl Marx's historical materialism posits that the material world is a product of human labor and social relations.