Few philosophical traditions traverse the history of Western thought as persistently as idealism. From Plato’s Theory of Forms to Hegel’s absolute system, passing through Berkeley’s immaterialism and Kant’s critical philosophy, the claim that reality is ultimately constituted or conditioned by thought, mind, or spirit resurfaces in radically diverse guises. This article examines the major strands of philosophical idealism — their premises, arguments, and internal differences — and the decisive critiques levelled against them by Marx, Russell, and Moore.
1. What is philosophical idealism?
In a broad sense, “idealism” designates any philosophical position that grants ontological or epistemological primacy to mind, spirit, ideas, or the structures of thought over matter or an independently existing external reality. Three fundamental senses should be distinguished from the outset:
- Ontological idealism: ultimate reality is mental or spiritual in nature; matter is appearance, copy, or product of spirit.
- Epistemological idealism: we only have access to mental representations; the “thing-in-itself” independent of mind is unknowable.
- Transcendental idealism: the subject’s a priori structures constitute the conditions of possibility of all experience — reality as we know it is phenomenal.
These three senses overlap but are not identical. Plato is an ontological idealist without being a subjectivist; Kant is a transcendental idealist who rejects ontological idealism; Berkeley is both a subjective and an ontological idealist. Understanding idealism therefore requires tracing each of its historical strands.
2. Plato: ontological idealism and the Theory of Forms
2.1 The sensible world as shadow
Plato (~428–348 BC) is the founder of Western idealism, although the term “idealism” is modern. His Theory of Ideas (or Forms) holds that full reality resides not in sensible objects — mutable, imperfect, perishable — but in eternal and immutable Forms (εἴδη), accessible only to reason. The sensible world is a copy (mimesis) or participation (methexis) in the intelligible world.
In the Republic (Books VI–VII), the celebrated Allegory of the Cave illustrates this ontology: prisoners mistake shadows projected on a wall for reality; the philosopher is the one who breaks free, ascends to the world illuminated by the Sun — the Form of the Good — and grasps that the shadows are mere appearances.
2.2 The hierarchy of being
For Plato, there is a rigorous ontological hierarchy:
- Form of the Good — the supreme principle, source of being and intelligibility.
- Intelligible Forms — Justice, Beauty, Equality, etc.
- Mathematical entities — intermediaries between the intelligible and the sensible.
- Sensible objects — imperfect copies of the Forms.
- Images and shadows — copies of copies.
2.3 Knowledge as recollection
If true reality is intelligible, how can the soul know it? Through recollection (anamnesis): the soul, before embodiment, contemplated the Forms; learning is remembering what it once beheld. Knowledge thus derives not from the senses but from reason turning toward the intelligible.
Platonic idealism is not subjectivist — the Forms exist independently of any individual mind. It is a realism of Forms: the intelligible is more real than the sensible.
3. George Berkeley: subjective idealism and esse est percipi
3.1 The elimination of matter
George Berkeley (1685–1753), Irish philosopher and Anglican bishop, pushed Locke’s empiricism to a radical conclusion: if all our knowledge derives from ideas (perceptions), and if we have no access to any material substance “behind” the ideas, then matter simply does not exist. His formula is famous: esse est percipi — “to be is to be perceived.”
3.2 The critique of primary qualities
Locke distinguished primary qualities (extension, shape, motion — belonging to the object) from secondary qualities (colour, taste, warmth — dependent on the subject). Berkeley argued, in A Treatise Concerning the Principles of Human Knowledge (1710), that this distinction is untenable: extension, shape, and motion are also ideas in the mind. The distinction between primary and secondary collapses; every perception is mental.
3.3 God as guarantor of continuity
If to be is to be perceived, what happens to objects when no one observes them? Berkeley’s answer: God perceives them continuously. The world is the language through which God communicates with finite minds. Thus, Berkeley’s immaterialism is not scepticism — it is, on the contrary, a defence of the reality of ideas and of God’s existence against materialism and atheism.
4. Immanuel Kant: transcendental idealism
4.1 The Copernican revolution in philosophy
Immanuel Kant (1724–1804) rejected both rationalist dogmatism and empiricist scepticism. His solution, set out in the Critique of Pure Reason (1781/1787), is transcendental idealism: we do not know things “as they are in themselves” (Ding an sich) but only as they appear to us (phenomena), shaped by the a priori structures of the cognising subject.
The so-called Kantian Copernican revolution reverses the relation between subject and object: it is not the subject that adapts to objects, but the objects of experience that conform to the conditions of possibility imposed by the subject — space and time (forms of sensibility) and the twelve categories of the understanding (causality, substance, etc.).
4.2 Phenomenon and thing-in-itself
The distinction between phenomenon and thing-in-itself is the axis of transcendental idealism:
- Phenomenon: the object as it appears to the subject, structured by a priori forms.
- Thing-in-itself (noumenon): reality as it is independently of every subjective condition — thinkable but unknowable.
Kant insists: his idealism is transcendental, not subjective. Phenomena are empirically real — objective within the framework of possible experience — even though they are transcendentally ideal (dependent on the subject’s conditions).
4.3 The limits of reason
In the Transcendental Dialectic, Kant demonstrates that when reason oversteps the bounds of possible experience, it inevitably produces illusions (antinomies). Traditional metaphysics — proofs of God’s existence, the immortality of the soul, freedom — is impossible as theoretical science. Yet freedom, God, and immortality reappear as postulates of practical reason, demanded by the moral law.
5. German Idealism: Fichte, Schelling, and Hegel
Kantian idealism left an unresolved tension: if the thing-in-itself is unknowable, why postulate it at all? The post-Kantians radicalised Kant’s position by eliminating the thing-in-itself and asserting that thought is being — nothing exists outside thought.
5.1 Fichte and the Absolute Ego
Johann Gottlieb Fichte (1762–1814) transformed Kantian philosophy into a rigorous system — the Doctrine of Science (Wissenschaftslehre). His single principle: the pure Ego (not the empirical individual self) is the unconditioned condition of all reality. The Ego is not a substance but an act — free self-positing.
Three theses articulate the system:
- The Ego posits itself (thesis) — originary freedom.
- The Ego opposes to itself a Non-Ego (antithesis) — the world as the necessary obstacle to freedom.
- Ego and Non-Ego mutually limit each other (synthesis) — determinate reality emerges from this interplay.
In his later phase (Berlin, 1800–1814), Fichte deepened the mystical dimension: the Ego is a manifestation of the Absolute (God); knowledge tends toward unity with the Absolute.
5.2 Schelling and the philosophy of identity
Friedrich Wilhelm Joseph Schelling (1775–1854) criticised Fichte: the Non-Ego cannot be a mere product of the Ego — nature possesses its own reality. For Schelling, nature is visible Spirit, and Spirit is invisible nature. Both are manifestations of the same Absolute — the undifferentiated identity of subject and object, ideal and real.
Schelling’s thought passed through multiple phases:
- Philosophy of Nature (~1797): nature as a living organism, a hierarchy of forms from matter to consciousness.
- Philosophy of Identity (~1801): the Absolute as the point of indifference between subject and object.
- Philosophy of Freedom (1809): in Investigations into the Essence of Human Freedom, Schelling argued that evil has positive reality — it is the perverse use of freedom, by which one isolates oneself from the whole.
- Positive Philosophy (late period): concrete existence is irreducible to any rational system — an anticipation of existentialism.
5.3 Hegel and absolute idealism
Georg Wilhelm Friedrich Hegel (1770–1831) constructed the most ambitious system in the history of philosophy. For Hegel, the real is rational and the rational is real. The Absolute is not a static substance but a process — Spirit (Geist) realises itself through history, nature, art, religion, and philosophy.
The Hegelian dialectic
Hegel’s dialectic is not a mechanical formula (thesis-antithesis-synthesis) but the internal movement of the Idea: every concept contains its negation; the negation is negated in an Aufhebung (sublation — overcoming that preserves). Thought progresses through contradiction and reconciliation:
- In the Phenomenology of Spirit (1807), Hegel traces the itinerary of consciousness from sense-certainty to Absolute Knowing — including the celebrated master-slave dialectic, where the master depends on the slave’s recognition and labour liberates the slave.
- In the Science of Logic (1812–16), the categories of thought develop dialectically: being → nothing → becoming → …
- In the Philosophy of Right (1821), Objective Spirit is realised as right, morality, and ethical life (family → civil society → state).
Absolute Spirit
Absolute Spirit manifests in three forms: art (sensuous intuition of the Absolute), religion (representation of the Absolute), and philosophy (concept of the Absolute). Philosophy is the supreme form of knowledge — the Absolute thinking itself.
Hegel’s celebrated dictum — “The Owl of Minerva takes flight only at dusk” — means that philosophy comprehends reality only after it has been consummated: reflection is retrospective.
6. British idealism: Bradley and McTaggart
In the second half of the nineteenth century, Hegelian idealism found unexpected receptivity in Britain, in opposition to the dominant empiricism and utilitarianism.
6.1 F. H. Bradley (1846–1924)
Francis Herbert Bradley, in Appearance and Reality (1893), argued that all relational and plural experience is mere appearance. Ultimate reality is the Absolute — a unified, indivisible, supra-relational experience. Relations, time, space, causality, and even the individual self are contradictory appearances. Only the Absolute, which transcends all distinctions, is fully real.
6.2 J. M. E. McTaggart (1866–1925)
John McTaggart Ellis McTaggart, in The Nature of Existence (1921–1927), defended a pluralist idealism: ultimate reality is composed of spiritual substances linked by love. His most celebrated contribution, however, is his argument for the unreality of time (1908), in which he distinguished two temporal series — the A-series (past-present-future) and the B-series (before-after) — and concluded that time, by generating insoluble contradictions, is unreal.
7. Critiques of idealism
7.1 Karl Marx: materialist inversion
Karl Marx (1818–1883) acknowledged Hegel’s merit in grasping reality as a dialectical process but accused him of having turned it “on its head.” In Marx’s famous formulation, it was necessary to invert the Hegelian dialectic: it is not Spirit that produces material reality, but material reality — the forces of production, the relations of production, the class struggle — that determines the forms of thought, law, religion, and philosophy (superstructure). Historical materialism is thus the direct negation of absolute idealism.
7.2 G. E. Moore: the refutation from common sense
G. E. Moore (1873–1958), in The Refutation of Idealism (1903), attacked the Berkeleyan principle esse est percipi. Moore argued that idealists confuse the act of perceiving with the object perceived: when I see a colour, the blue I perceive is not identical with my sensation of blue. Consciousness is transparent — it points to something distinct from itself. This distinction between act and object undermined the central premise of subjective idealism.
In A Defence of Common Sense (1925) and Proof of an External World (1939), Moore maintained that propositions such as “hands exist” and “material objects exist” are more certain than any philosophical argument purporting to deny them.
7.3 Bertrand Russell: logical atomism against monism
Bertrand Russell (1872–1970) abandoned the British idealism of his formation to found, alongside Moore, the analytic tradition. His logical atomism held that reality is composed of independent “atomic facts” — a position diametrically opposed to Bradley’s monism of the Absolute. Russell argued that logical analysis dissolves the pseudo-problems generated by linguistic confusion and that philosophy should follow the model of science, not metaphysical speculation.
In A History of Western Philosophy (1945), Russell acknowledged the greatness of German idealism but judged its systems to suffer from linguistic ambiguity and illegitimate inferences.
8. Legacy and continuing relevance
Despite the devastating critiques of the twentieth century, idealism continues to exert decisive influence:
- Husserl’s phenomenology takes up Kant’s transcendental theme — consciousness as the giver of meaning.
- Gadamer’s hermeneutics is unthinkable without Hegel.
- Contemporary philosophy of mind debates theses that echo Berkeley and Kant: the constitutive role of consciousness, the limits of access to the “in-itself,” the hard problem of consciousness.
- Analytic idealism — recently defended by philosophers such as Bernardo Kastrup and Timothy Sprigge — seeks to rehabilitate idealism within the analytic framework.
The fundamental idealist question — can there be reality entirely independent of mind? — remains alive and unresolved.
Conclusion
Philosophical idealism is not a single doctrine but a family of positions sharing the conviction that mind, spirit, or the structures of thought play a constitutive role in reality. From Plato to Hegel, each strand responded to the specific problems of its time: the relation between appearance and essence, between subject and object, between thought and being. The critiques of Marx, Moore, and Russell did not eliminate idealism — they redefined the terms of the debate. To understand this tradition is to understand one of the most persistent and fertile threads in the entire history of philosophy.
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