Modern Western philosophy: rationalism, empiricism and Kant

In short

Modern Western philosophy runs from Descartes (1596-1650) to Kant (1724-1804) and asks one governing question: what can the human mind know, and on what authority? Rationalists (Descartes, Spinoza, Leibniz) held that reason alone, working from self-evident premises, yields certain knowledge of reality; empiricists (Locke, Berkeley, Hume) held that all content of the mind comes from sense experience. Kant argued both were half right and produced the critical philosophy, which has shaped every later movement in ethics, science and political theory. In India it is a compulsory paper in BA/MA Philosophy (UGC CBCS) and carries heavy weight in UGC-NET Philosophy (Unit on Western Philosophy), UPSC Civil Services Philosophy optional Paper I, and state PSC philosophy papers; the reasoning skills - distinguishing premise from conclusion, analytic from empirical claims - are directly tested in placement aptitude, logical-reasoning and critical-thinking rounds, and in law entrance (CLAT, AILET) legal-reasoning sections.

Explained

The problem that creates modern philosophy

• Historical break: the Scientific Revolution (Copernicus, Galileo, Newton) made the medieval Aristotelian-scholastic picture untenable; authority (Church, Aristotle) was replaced by method as the ground of knowledge.
• Core question (epistemology becomes first philosophy): before asking what exists, ask how we can know anything at all. Metaphysics is now answerable only after epistemology.
• Three standing disputes:
  1. Source of knowledge - reason (a priori) or experience (a posteriori)?
  2. Innate ideas - does the mind come furnished, or is it blank at birth?
  3. Substance - how many ultimate kinds of thing exist (dualism, monism, pluralism)?
• Key distinctions used throughout:
  • A priori - known independently of experience (7+5=12). A posteriori - known through experience (water boils at 100 °C at sea level).
  • Analytic - predicate contained in the subject, true by meaning ("all bachelors are unmarried"). Synthetic - predicate adds new information ("the table is brown").
  • Necessary (could not be otherwise) versus contingent (happens to be so).
  • Deduction (conclusion certain if premises true) versus induction (conclusion probable, generalising from instances).

Rationalism: Descartes, Spinoza, Leibniz

Descartes - the method of doubt

• Aim: find an indubitable foundation ("Archimedean point") for knowledge. Works: *Discourse on Method* (1637), *Meditations on First Philosophy* (1641).
• Four rules of method: accept nothing not clearly and distinctly true; divide difficulties into parts (analysis); proceed from simple to complex (synthesis); review completely (enumeration).
• Three waves of methodic (hyperbolic) doubt: (1) senses sometimes deceive, so all sense reports are suspect; (2) the dream argument - no sure mark distinguishes waking from dreaming, so the whole external world is doubtful; (3) the evil demon (malin génie) hypothesis - even mathematics could be systematically falsified by a deceiver.
• Cogito ergo sum: the one thing the deceiver cannot falsify is that I, doubting, exist. Note: it is a performative self-verification each time it is thought, not a syllogism with a suppressed premise.
• What am I? A *res cogitans* - a thinking thing, essence = thought, known better than body. The wax example shows even bodily knowledge comes from intellect, not sense or imagination.
• Criterion of truth: whatever is clearly and distinctly perceived is true - but this needs God's non-deception to be guaranteed, giving the famous Cartesian circle (clear ideas prove God; God validates clear ideas).
• Proofs of God: (a) causal/trademark - my idea of an infinite being needs an adequate cause; (b) ontological - existence belongs to the essence of a supremely perfect being; (c) conservation - something must sustain me moment to moment.
• Dualism and interactionism: two created substances - mind (thinking, unextended, indivisible) and matter (extended, unthinking, divisible). Mind-body problem: how can an unextended thing cause motion in an extended one? Descartes's answer (interaction at the pineal gland) is universally judged a failure and violates conservation principles.
• Contribution and limits: founded the subject-centred turn and analytic geometry; criticised for the circle, for the cogito's leap from "thinking occurs" to "I, a substance, think" (Hume, Kant, Lichtenberg), and for leaving the external world unproved without theology.

Spinoza - substance monism (*Ethics*, 1677, written in geometrical order: definitions, axioms, propositions, QED)

• One substance: substance is "that which is in itself and conceived through itself"; since it is self-caused and infinite, there can be only one - Deus sive Natura (God or Nature). Pantheism, not theism.
• Attributes and modes: substance has infinite attributes, of which we know two - thought and extension; particular minds and bodies are *modes* (modifications) of these. Mind and body are one thing under two descriptions - double aspect theory / parallelism, dissolving Descartes's interaction problem.
• Natura naturans (Nature as active, creating) versus natura naturata (Nature as the system of modes produced).
• Determinism and freedom: everything follows from God's nature with geometrical necessity; freedom is not uncaused choice but acting from adequate knowledge of necessity. Freedom = self-determination, not contingency.
• Three kinds of knowledge: (1) imagination/opinion from hearsay and casual experience (inadequate); (2) reason - common notions, adequate ideas; (3) scientia intuitiva - seeing things *sub specie aeternitatis*, the highest, yielding the "intellectual love of God" and *beatitudo*.
• Conatus: every thing strives to persevere in its being; emotions (affects) are bondage when passive, virtue is action from reason.

Leibniz - monadic pluralism (*Monadology*, *Discourse on Metaphysics*, *New Essays*)

• Monads: infinitely many simple, unextended, indivisible immaterial substances - "windowless" (no causal traffic between them), each a point of perception (*perception* and *appetition*). Matter and extension are well-founded appearances of aggregates of monads.
• Pre-established harmony: monads appear to interact because God synchronised them at creation, like perfectly set clocks - his alternative to Cartesian interaction and to occasionalism (Malebranche, where God acts at each instant).
• Two great principles: (1) Principle of Contradiction/Identity - governs necessary truths, whose opposite is impossible, provable by analysis in finite steps; (2) Principle of Sufficient Reason - nothing is without a reason; governs contingent truths of fact, whose analysis is infinite and known fully only to God.
• Truths of reason (necessary, analytic, true in all possible worlds) versus truths of fact (contingent, depend on God's choice). Identity of indiscernibles: no two substances are exactly alike.
• Best of all possible worlds: God, being good and bound by sufficient reason, chose the world with maximum variety and order - the basis of his Theodicy, satirised by Voltaire in *Candide*.
• Petites perceptions: unconscious minute perceptions - an anticipation of the unconscious and of the continuity of mental life; used against Locke's claim that nothing is in the mind without being noticed.

Empiricism: Locke, Berkeley, Hume

Locke (*An Essay concerning Human Understanding*, 1690)

• Anti-innatism: the mind is a *tabula rasa* (white paper); no innate speculative or practical principles - evidence: children, idiots, cultural variety; an idea "in the mind but not perceived" is unintelligible.
• Two sources of ideas: sensation (outer sense) and reflection (inner sense, the mind's awareness of its own operations). Nothing enters otherwise - *nihil in intellectu quod non prius in sensu*.
• Simple and complex ideas: simple ideas are received passively and cannot be invented; the mind actively combines, compares and abstracts them into complex ideas (modes, substances, relations).
• Primary qualities (solidity, extension, figure, motion, number) resemble their causes and are in the object; secondary qualities (colour, sound, taste, smell, heat) are powers in the object to produce sensations, and do not resemble anything in it - the representative theory of perception (ideas are the immediate objects; things are known only mediately).
• Substance: "something I know not what" - an unknown substratum supporting observed qualities; an admission empiricism cannot cash out.
• Three grades of knowledge: intuitive (self), demonstrative (God, mathematics), sensitive (external things - least certain). Real and nominal essence distinguished.
• Political theory (*Two Treatises*, 1689): natural rights to life, liberty and property; government by consent and trust; right of revolution - a direct source of the American Declaration of Independence and of liberal constitutionalism, including Fundamental Rights thinking.

Berkeley (*Principles of Human Knowledge* 1710; *Three Dialogues* 1713)

• Esse est percipi - to be is to be perceived (for a thing); to be is to perceive (for a spirit). Subjective idealism / immaterialism: matter is a contradiction; only minds and their ideas exist.
• Argument against matter: Locke's own premises entail it. If all we ever have are ideas, an unperceived material substratum is an idea of nothing; and primary qualities are as mind-dependent as secondary (extension and motion are relative to the perceiver). The master argument: you cannot conceive an unconceived object.
• Attack on abstract general ideas: there is no idea of a triangle neither scalene nor isosceles; generality belongs to the use of a particular idea as a representative sign.
• Why things do not vanish when unobserved: they are continuously perceived by God, whose will is the cause of the orderly, involuntary, vivid ideas we call real; the "laws of nature" are regularities in God's ideas. Science becomes the description of regularity, not of hidden causes.
• Motivation: defend common sense and religion against scepticism and materialism - "I side in all things with the mob."

Hume (*A Treatise of Human Nature* 1739-40; *Enquiry concerning Human Understanding* 1748)

• Impressions and ideas: all mental contents are either impressions (lively, original) or ideas (faint copies). Copy principle: every simple idea derives from a corresponding impression - the test that makes a term meaningful.
• Hume's fork: all objects of enquiry are either relations of ideas (a priori, demonstrative, mathematics - denial is contradictory) or matters of fact (a posteriori, probable, denial always conceivable). Anything that is neither is "sophistry and illusion".
• Causation: we observe only (1) contiguity, (2) temporal priority, (3) constant conjunction - never a necessary connection. Causation as necessity is a projection of the mind's custom/habit; the idea of necessity arises from the felt determination of the imagination to pass from cause to effect.
• Problem of induction: inference from observed to unobserved presupposes the uniformity of nature, which can be proved neither demonstratively (its denial is not contradictory) nor empirically (that would be circular). Not scepticism about practice - "custom is the great guide of human life" - but scepticism about its rational justification.
• No self as substance: introspection finds only a "bundle" of perceptions in flux; the self is a fiction built by memory and resemblance. Personal identity is a question about the ideas, not an observed substance.
• External world: belief in continuing bodies rests on constancy and coherence of impressions worked up by imagination, not on reason.
• Ethics: the is-ought gap - no "ought" follows from any set of "is" statements; "reason is wholly inactive" and "a slave to the passions"; morality is founded on sentiment (approval, sympathy) - moral sentimentalism, the root of emotivism.
• Religion: critique of the design argument (*Dialogues concerning Natural Religion*, published posthumously 1779) and of miracles (*Enquiry*, Section X); a miracle report is credible only if its falsehood would be more miraculous.
• Significance: consistent empiricism ends in scepticism about causation, substance, self and induction - the result that "interrupted Kant's dogmatic slumber".

Kant: the critical philosophy

• Kant's problem: Hume showed experience cannot yield necessity; yet Newtonian science and mathematics clearly are necessary and informative. So how is synthetic a priori knowledge possible?
• The fourfold classification: analytic a priori (logic, definitions); synthetic a posteriori (ordinary empirical fact); synthetic a priori (mathematics, pure natural science, metaphysics - the problem class); analytic a posteriori (empty - nothing can be both).
• Copernican revolution in philosophy: instead of the mind conforming to objects, objects of experience conform to the mind's forms. Knowledge is a joint product: "thoughts without content are empty, intuitions without concepts are blind."
• Sensibility and understanding: the mind receives a manifold through sensibility, ordered by the a priori forms of intuition - space and time (space is the form of outer sense, time of inner sense; both are ideal, not properties of things in themselves), and then synthesises it under the categories of the understanding.
• Structure of the Critique of Pure Reason (1781, 2nd edn 1787):
  • Transcendental Aesthetic - space and time as forms of intuition; explains the synthetic a priori status of geometry and arithmetic.
  • Transcendental Analytic - the twelve categories in four triads: Quantity (unity, plurality, totality); Quality (reality, negation, limitation); Relation (inherence/substance, causality, community); Modality (possibility, existence, necessity), derived from the forms of judgement (metaphysical deduction).
  • Transcendental Deduction - the categories apply necessarily because without them the manifold could not be unified in one consciousness; the supreme principle is the transcendental unity of apperception ("I think" must accompany all my representations). The Schematism supplies the temporal rules that let pure categories apply to sensible content.
  • Transcendental Dialectic - reason's unavoidable illusions when categories are used beyond experience.
• Phenomena and noumena: we know appearances (phenomena) as structured by our forms - and this is genuine objective knowledge, not illusion; the thing in itself (*Ding an sich*, noumenon) is a limiting concept, thinkable but unknowable. Kant's position is transcendental idealism combined with empirical realism.
• The three illusions of reason (ideas of soul, world, God):
  • Paralogisms - the rational psychology of a substantial, simple, immortal soul mistakes the formal unity of apperception for an intuited object.
  • Antinomies - four pairs of equally provable contradictory theses about the world: (1) the world has/has not a beginning and limit in space; (2) everything is/is not composed of simple parts; (3) there is/is not freedom besides natural causality; (4) there is/is not a necessary being. Resolution: the first two are both false (the world as a whole is not a given object); the third and fourth can both be true - a person may be determined as phenomenon and free as noumenon.
  • Ideal of pure reason - the ontological, cosmological and physico-theological proofs of God all fail; against the ontological proof, "existence is not a real predicate" (a hundred real thalers contain no more than a hundred possible ones).
• Practical philosophy (*Groundwork* 1785, *Critique of Practical Reason* 1788): the only unconditional good is the good will; morality is a categorical imperative, not a hypothetical (means-end) one. Formulations: (a) universal law - act only on a maxim you can will as a universal law; (b) humanity as an end - never treat a person merely as a means; (c) autonomy/kingdom of ends - act as a legislating member of a realm of rational ends. Duty versus inclination; freedom, God and immortality return as postulates of practical reason - not known, but rationally presupposed. Kant's dictum: "I had to deny knowledge in order to make room for faith."
• Third Critique (*Critique of Judgement*, 1790): aesthetic judgement is subjective yet claims universality - "disinterested" pleasure, the beautiful and the sublime, purposiveness without purpose; teleological judgement is regulative for biology, not constitutive.
• Criticisms: the thing in itself is affected by causality though causality is said to apply only to appearances (Jacobi's objection); non-Euclidean geometry and relativity undermined the claimed apriority of Euclidean space and absolute time; the table of categories is charged with being arbitrarily drawn from Aristotelian logic; Hegel rejected the unknowable noumenon as incoherent and the formalism of the moral law as empty.

Comparisons, lineages and later influence

• Rationalism's strength: accounts for necessity, universality and mathematics. Weakness: gives no test to stop reason spinning out unverifiable systems; "clear and distinct" is not public.
• Empiricism's strength: anchored in observation, consonant with experimental science, deflationary about metaphysics. Weakness: cannot justify induction, mathematics' necessity, substance or self - ends in scepticism.
• Kant's synthesis: experience requires both, and metaphysics is limited to the conditions of possible experience. Cost: things in themselves are forever out of reach.
• Lineage forward: Kant → German Idealism (Fichte, Schelling, Hegel) → Marx; Hume → Comte, Mill, logical positivism (Vienna Circle verification principle), Russell and Ayer; Hume's induction problem → Popper's falsificationism; Descartes's subject → Husserl's phenomenology; Berkeley/Hume on self → debates in philosophy of mind and cognitive science; Kant's innate structure → Chomsky's universal grammar and Piaget's schemas; Kant's dignity formula → human-rights doctrine and bioethics consent norms.
• Indian comparative notes (examined in UGC-NET/UPSC comparative papers): Cārvāka is empiricist in accepting only *pratyakṣa* (perception) as *pramāṇa* and so faces Hume's induction difficulty about *anumāna*; Advaita Vedānta's *māyā*/*vyavahārika-pāramārthika* distinction invites comparison with the phenomenon-noumenon distinction, though Śaṅkara affirms knowable Brahman while Kant denies knowledge of the noumenon; Buddhist *anātmavāda* and the doctrine of momentariness parallel Hume's bundle self; Nyāya's realist *pramāṇa* theory contrasts with both.

Key points

• Rationalism - reason is the source of substantive knowledge; truths are necessary, a priori, deductive (Descartes, Spinoza, Leibniz).
• Empiricism - all ideas come from experience; factual truths are contingent and a posteriori (Locke, Berkeley, Hume).
• Methodic doubt - Descartes's tool: doubt everything doubtful to find what cannot be doubted; not scepticism but a search for certainty.
• Cogito ergo sum - self-verifying each time it is thought; the first indubitable truth, and the foundation of the modern subject.
• Cartesian dualism - two substances, mind (thought) and body (extension); its unsolved mind-body interaction problem drives Spinoza, Leibniz and Malebranche.
• Spinoza's monism - one substance, *Deus sive Natura*, with infinite attributes; mind and body are one thing under thought and extension.
• Leibniz's monads - infinitely many windowless simple substances harmonised in advance by God; PSR governs contingent truth.
• Tabula rasa - Locke's blank-slate mind; no innate ideas, only sensation and reflection.
• Primary/secondary qualities - primary are in the object and resemble our ideas; secondary are powers producing colour, taste, sound in us.
• Esse est percipi - Berkeley: matter does not exist; things are collections of ideas, kept in being by God's perception.
• Copy principle - Hume: every simple idea traces to an impression; a term with no impression behind it is meaningless.
• Hume's fork - relations of ideas (demonstrable, a priori) versus matters of fact (probable, a posteriori); nothing else is knowledge.
• Constant conjunction - what we actually observe; necessary connection is a habit of mind, not a feature of the world.
• Problem of induction - the uniformity of nature cannot be proved without assuming it; reason cannot justify any prediction.
• Is-ought gap - no normative conclusion follows from purely factual premises; morality rests on sentiment.
• Synthetic a priori - necessary and informative judgement (7+5=12; every event has a cause); its possibility is Kant's central question.
• Copernican revolution - objects conform to the mind's forms (space, time, categories), not the reverse; hence objective yet mind-dependent experience.
• Phenomenon versus noumenon - we know appearances, never the thing in itself; antinomies arise when categories stray beyond experience.
• Categorical imperative - unconditional moral law of reason; universalisability plus treating humanity always as an end.

Examples

Worked case 1 - running Descartes's doubt in order (what survives each wave)

Step 1: sense-illusion removes particular sense reports but not the general existence of bodies. Step 2: the dream argument generalises - if there is no mark distinguishing dreaming from waking, the whole external world including my body goes. Step 3: the demon reaches even mathematics, which dreaming could not touch. Step 4: but deception presupposes a deceived thinker; the act of doubting entails a doubter. Conclusion: the cogito is the only survivor, hence the foundation. Note the inference is to *thinking occurring*, not yet to an immaterial *substance* - that further step is where Hume and Kant attack.

Worked case 2 - classifying judgements on Hume's fork and Kant's grid

Step-by-step reasoning for row 3: (i) analyse the subject "7 + 5" - it contains only the two numbers and the operation of combining; (ii) no amount of unpacking yields the specific concept "12"; (iii) so the predicate adds information - synthetic; (iv) yet it is known with necessity and without any particular counting of objects - a priori; (v) therefore synthetic a priori, which on Hume's fork is impossible. Kant's explanation: arithmetic rests on the pure intuition of time (successive synthesis), geometry on that of space. Exam caution: post-Frege, most logicians hold arithmetic is analytic or logically true, and relativity has displaced Kant's claim about Euclidean space - state this as the standard criticism.

Worked case 3 - the induction problem on a real inference

• Claim: "The next monsoon will bring rain to Kerala in June."
• Evidence: it has done so in every recorded year.
• Step 1: the inference is from observed cases to an unobserved one.
• Step 2: this needs the premise "nature is uniform / the future resembles the past."
• Step 3: can that premise be demonstrated? No - "the monsoon fails entirely" is conceivable, not self-contradictory, so it is not a relation of ideas.
• Step 4: can it be established from experience? Only by arguing that past uniformity continued, which assumes the premise - circular.
• Step 5: therefore no rational justification; the belief is produced by custom acting on the imagination.
• Consequence for science: Popper's reply - do not justify induction, use falsification: a theory is corroborated by surviving severe tests, never proved. IMD monsoon forecasts honour this in practice by issuing probabilistic, revisable forecasts rather than certainties.

Worked case 4 - applying the categorical imperative to a real decision Situation: a candidate considers inflating experience on a résumé for a campus placement.

1. State the maxim: "Whenever it helps me get a job, I will misstate my experience."
2. Formula of universal law: universalise it - if every candidate misstated experience, employers would stop believing résumés, so the very practice that makes the lie effective collapses. The maxim is self-defeating when universalised - a contradiction in conception; the duty not to lie is therefore a perfect duty.
3. Formula of humanity: the lie uses the recruiter merely as a means to a salary, bypassing their rational consent; it treats a person as an instrument.
4. Verdict: impermissible absolutely - not because of bad consequences (that would be hypothetical/prudential reasoning) but because the maxim cannot be willed as law.
5. Contrast with a utilitarian answer: a consequentialist would weigh the expected gain against the chance of being caught and the harm to others - showing exactly why Kant's test is non-consequentialist. Contrast with an imperfect duty: "never help others in need" is universalisable without contradiction in conception, but no rational being could *will* it (contradiction in the will), so beneficence is an imperfect duty - latitude in how and when, but it may not be wholly refused.

Worked case 5 - comparison table for revision

Where it is used

• Teaching and research: BA/MA Philosophy departments across Indian universities (Delhi, JNU, Jadavpur, Madras, Pune, BHU), UGC-NET/JRF qualification and PhD entrance; Western Philosophy is a standard CBCS core course.
• Competitive examinations: UPSC Civil Services Philosophy optional Paper I (Section A is largely this syllabus); state PSC philosophy papers; essay and ethics papers draw on Kant's deontology versus Mill's utilitarianism for the GS-IV Ethics, Integrity and Aptitude paper.
• Law, policy and regulation: Locke's natural rights and consent theory underlie constitutional rights reasoning; Kant's dignity formula appears in Indian jurisprudence on autonomy and privacy (the right-to-privacy and personal-autonomy line of Supreme Court cases) and in informed-consent norms used by ICMR ethics guidelines and institutional ethics committees.
• Corporate ethics and compliance roles: deontological (rule/duty) versus consequentialist frameworks are the explicit vocabulary of codes of conduct, whistleblower policy and CSR committees under the Companies Act, 2013; ethics officers and compliance analysts are expected to argue in these terms.
• AI, data and cognitive science: the rationalism-empiricism dispute recurs as nativism versus learning-from-data (symbolic AI versus machine learning); Kantian consent and dignity arguments shape data-protection principles reflected in the Digital Personal Data Protection Act, 2023 (purpose limitation, notice and consent).
• Content, editorial and civil-services coaching work: philosophy graduates write curriculum, test items and explainers; publishing houses and ed-tech firms hire for critical-reasoning item writing, where the analytic/synthetic and deduction/induction distinctions are the working tools.

Interview questions

• Q: What exactly divides rationalism from empiricism? A: The source and warrant of knowledge - rationalists hold that reason alone can yield substantive necessary truths about reality, empiricists that all content and all factual warrant come from sense experience. Both agree knowledge is possible; they disagree on its foundation.
• Q: Is "cogito ergo sum" an inference? A: Descartes presents it as immediately evident rather than a syllogism: the act of thinking cannot occur without a thinker, so it is self-verifying whenever thought. Lichtenberg's objection is that it licenses only "thinking is occurring", not "I, a substance, am".
• Q: What is the Cartesian circle? A: Clear and distinct ideas are certified as true by God's non-deception, but God's existence is itself proved using clear and distinct ideas - so the criterion and its guarantee support each other circularly.
• Q: How do Spinoza and Leibniz each avoid Descartes's mind-body problem? A: Spinoza denies there are two things to interact - mind and body are one substance under two attributes; Leibniz denies interaction altogether, explaining correlation by a pre-established harmony set up by God.
• Q: Why does Berkeley think Locke's position collapses into idealism? A: If the immediate objects of awareness are always ideas, an unperceived material substratum can never be experienced or coherently conceived; and primary qualities are as perceiver-relative as secondary ones, so no mind-independent matter is left.
• Q: State Hume's analysis of causation in one sentence. A: All we observe is the contiguity, succession and constant conjunction of events; the "necessary connection" is a determination of the imagination produced by custom, projected onto the world.
• Q: Why can't induction be justified? A: It requires the uniformity of nature, whose denial is not self-contradictory (so no demonstration) and whose support from past regularity presupposes the very principle at issue (so the empirical argument is circular).
• Q: What is a synthetic a priori judgement and give two examples? A: One that is both informative (predicate not contained in the subject) and necessary/independent of experience - Kant's examples are "7 + 5 = 12" and "every event has a cause". Note that most philosophers after Frege deny arithmetic is synthetic.
• Q: What is Kant's Copernican revolution? A: Rather than assuming knowledge must conform to objects, Kant holds that objects of experience conform to the mind's a priori forms of intuition and categories - which is why we can know some things about objects in advance of experiencing them.
• Q: If we only know phenomena, is Kant a sceptic? A: No. He is an empirical realist: knowledge of appearances is fully objective because the forms are universal and necessary for all rational beings; only the thing in itself is beyond knowledge, and it is thinkable, not nonsense.
• Q: Why does Kant reject the ontological argument? A: Because "exists" is not a real predicate that adds to a concept - the concept of a hundred real coins has no more content than a hundred possible ones, so existence can never be derived from a definition.
• Q: How would Kant and Hume differ on a manager asked to tell a small lie that helps everyone? A: Hume would ask whether the act is approved by sympathy and whether it promotes utility and agreeable sentiment; Kant would test the maxim for universalisability and find lying impermissible regardless of benefit, since it treats others merely as means.

Common mistakes

• Treating methodic doubt as scepticism - Descartes doubts as a method to reach certainty; Hume's is the sceptical conclusion.
• Writing that Hume denied causation. He denied only the observed *necessary connection*; he fully accepts causal belief as the guide of life and gives rules for judging causes.
• Calling Berkeley a sceptic or a solipsist - he defends common sense against scepticism, and God's perception plus other spirits rule out solipsism.
• Saying Kant denied that we know anything, or that phenomena are "mere illusion" - Kant affirms objective knowledge of appearances; the restriction is only to the thing in itself.
• Confusing analytic/synthetic with a priori/a posteriori, and therefore missing why synthetic a priori is the hard case; also confusing the categorical imperative with the Golden Rule (Kant explicitly rejects the Golden Rule as it depends on contingent inclinations).
• Attributing the innate-ideas claim to Locke, or the *tabula rasa* to Leibniz; and crediting Hume with the term "is-ought gap" as an argument for relativism rather than against deriving values from facts.
• Narrating lives and dates instead of arguing; exam answers need the premises, the inference and one named objection - and should separate the philosopher's claim from the standard criticism rather than merging them.

Related topics

Greek philosophy: Socrates, Plato and Aristotle · Metaphysics and epistemology · Existentialism and phenomenology · Ethics: deontology, utilitarianism and virtue ethics · Logic: deduction, induction and fallacies · Philosophy of science: Popper, Kuhn and the demarcation problem.