Plato (the Theory of Forms)

The working‑memory excerpts for The Republic focus on justice, the structure of the city‑state, education, and the role of poetry, but they do not contain a passage that spells out Plato’s doctrine of the Forms (the timeless, non‑material archetypes of which sensible things are imperfect copies). Because that material is absent, I cannot quote a specific passage for Plato’s view. What is missing: a summary or quotation of the Form‑theory (e.g., the Form of the Good, the Form‑world as the true reality behind appearances).

Kant (the “thing‑in‑itself”)

Kant distinguishes between the world as it appears to us (phenomena) and the world as it is independently of our cognition (the “thing‑in‑itself”). This distinction underlies his claim that we can never know the thing‑in‑itself directly; we only ever have knowledge of the way objects are given to us through the a‑priori forms of sensibility (space and time) and the categories of the understanding.

“Kant argues that the concept of freedom, or liberty, is not necessarily contradictory, even if it cannot be directly cognized through intuition… the distinction between sensible and intellectual representations… allows us to understand the limits of our knowledge.”  “Kant argues that his critical system of metaphysics has a positive value, not just a negative one, as it prevents speculative reason from overstepping its bounds and allows practical reason to operate freely… By distinguishing between things as they appear to us (phenomena) and things as they are in themselves, Kant's system enables us to reconcile… the freedom of the human will and its subjection to natural necessity.” 

These passages show that for Kant the world we perceive is real insofar as it is structured by the categories and the forms of intuition, but that the ultimate “thing‑in‑itself” remains unknowable.

Schopenhauer (the world as representation / “idea”)

Schopenhauer adopts Kant’s epistemic framework and then adds his own metaphysical claim: the world we experience is a representation (Vorstellung), i.e., an “idea” that exists only as it is mediated by the subject’s cognition. The underlying reality, which he calls the “Will,” is a blind, striving force that cannot be known directly.

“Schopenhauer argues that the world is fundamentally an ‘idea’ or a perception of a perceiver… everything that exists is conditioned by the subject and exists only for the subject… the world is not an objective reality independent of our perception.”  “According to Schopenhauer, the world has two fundamental aspects: the world as idea (the world of objects and phenomena) and the world as will (the underlying, unknowable force that drives reality)… our understanding of the world is limited to the world as idea, which is shaped by the principle of sufficient reason.” 

Thus, for Schopenhauer the world we perceive is real as a mental construction, but it is not the ultimate reality; it is the “representation” of the Will.

Comparison

Bottom line

All three thinkers agree that the world of perception is not the final, ultimate reality. Plato posits a higher, non‑sensible realm of Forms; Kant maintains that we can know only the phenomenal world, with the thing‑in‑itself forever inaccessible; Schopenhauer reduces the phenomenal world to a representation shaped by the subject, while locating the true essence in the blind Will. Because the memory does not contain a passage on Plato’s Forms, the comparison relies on the Kant and Schopenhauer material that is present.

None of the three philosophers you have in mind—Kant, Schopenhauer, or Hegel—assert that ordinary perception outright deceives us.

• Immanuel Kant holds that our sensible experience is always already shaped by the a‑priori forms of intuition (space, time) and the categories (causality, etc.). What we see are phenomena—the way objects appear under those conditions—not the “thing‑in‑itself.” This is a claim about the limits of perception, not that perception is a deliberate illusion. (see the Kant‑summaries in the “Critique of Pure Reason” section.)
• Arthur Schopenhauer goes a step further and says that the world we know is “the world as idea” (i.e. a representation), while the “will” is the true thing‑in‑itself. Again, this shows that ordinary perception gives us only a representation, not that it actively deceives us.  § 151 makes this clear:

“Phenomenal existence is idea and nothing more… the will alone is a thing in itself.” 

• Georg W. F. Hegel rejects the Kantian/Schopenhauer dualism altogether, calling the “thing‑by‑itself” an empty abstraction that ignores a thing’s concrete self‑development. For Hegel the thing is always already in a process of dialectical unfolding; there is no hidden, deceptive veil that ordinary perception must pierce. (see Hegel’s passage on the “thing‑by‑itself.”) So while all three agree that ordinary perception does not give us direct access to the ultimate reality, none of them say that perception deceives us in the sense of presenting a false picture. Their critiques are about epistemic limitation, not about deliberate illusion.

Plato’s discussion of justice begins with the very literal formula “paying one’s debts,” but over the course of The Republic the dialogue shows why this definition is inadequate and how it is gradually transformed into a much richer account of the soul and the city.

1. Cephalus’s literal definition – The conversation opens in Cephalus’s house, where Socrates asks him what justice is. Cephalus replies that it is “to speak the truth and to pay one’s debts” . This view treats justice as a matter of external obligations and honest speech.
2. Socratic criticism of the literal view – Socrates immediately points out a problem: if a friend who is temporarily insane asks for a weapon that he will use to harm himself, returning the weapon would be “paying a debt” but would be unjust. The dialogue therefore shows that a simple “debt‑paying” rule cannot handle cases where the object of the debt would cause harm .
3. Polemarchus’s refinement – “give each his due” – After Cephalus departs, Polemarchus proposes that justice means “giving each his due,” i.e. doing good to friends and harm to enemies. This moves the discussion from a formal rule to a relational one, but Socrates soon questions whether harming anyone can be part of a just life, because harming corrupts the soul .
4. Further correction – the good‑friend, good‑enemy distinction – The dialogue later recognises that the terms “friend” and “enemy” must be qualified: a true friend is one who is genuinely good, and a true enemy is one who is genuinely bad. Justice therefore cannot be about harming anyone arbitrarily; it must be about doing good to those who are good and not harming the good .
5. Thrasymachus’s challenge – justice as the interest of the stronger – Thrasymachus attacks the whole project by claiming that justice is simply whatever serves the rulers’ interest. Socrates counters by showing that true expertise (e.g., medicine, piloting) serves the thing it governs, not the expert, and that rulers can be mistaken about their own interest, so justice cannot be reduced to “the interest of the stronger” .
6. Glaucon and Adeimantus – the demand for intrinsic value – The brothers ask Socrates to prove that justice is good in itself, not merely for its external rewards. This pushes the discussion beyond any external “debt‑paying” or power‑based definition toward a view of justice as a virtue of the soul .
7. Socratic synthesis – “doing one’s own business” – In the later books (IV–VIII) Socrates offers the final definition: justice is the principle of “doing one’s own business.” In the city each class (rulers, auxiliaries, producers) performs its natural function; in the individual the rational, spirited, and appetitive parts are ordered so that each fulfills its proper role without interfering with the others. This internal non‑interference generates harmony and the other virtues, making justice an intrinsic good of the soul rather than a mere external transaction .

Summary of the development

• Starting point: justice = literal repayment of debts (Cephalus).
• First revision: justice = giving each his due (Polemarchus).
• Second revision: the “due” must be based on true goodness, not on mistaken friendships or enmities (later Polemarchus).
• Third revision: justice is not simply the interest of the powerful (Thrasymachus refuted).
• Fourth revision: justice must be intrinsically valuable, not merely instrumental (Glaucon & Adeimantus).
• Final synthesis: justice = the harmonious ordering of parts, each doing its proper work, both in the city and in the soul (Socratic synthesis). Thus, Plato’s definition evolves from a narrow, external accounting of obligations to a comprehensive moral architecture that secures internal harmony and the good life.

Schopenhauer uses the term “the blind will” to denote the thing‑in‑itself that underlies every phenomenon. For him the Will is not a rational, purposeful agency but an irrational, unconscious striving that drives all of nature and human life. It is “blind” because it operates without knowledge, intention or moral direction; it simply wants and exerts force, and everything we experience as objects, bodies, or events is merely the manifestation of this underlying will‑force in the form of space, time and causality.

Thus, when Schopenhauer speaks of “the blind will” he is pointing to the fundamental, non‑conceptual force that is the true reality behind the world of ideas, a force that can never be known directly but only inferred from the way phenomena appear to us.