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Incapable of being measured against a common standard. The presumed incommensurability of individual human pleasures is sometimes raised as an objection against hedonistic versions of utilitarianism. Feyerabend and Kuhn suppose that rival scientific theories are incommensurable if neither can be fully stated in the vocabulary of the other.

Recommended Reading: Nola J. Heidlebaugh, Judgement, Rhetoric, and the Problem of Incommensurability (South Carolina, 2001); Howard Sankey, The Incommensurability Thesis (Avebury, 1994); and Incommensurability, Incomparability, and Practical Reason, ed. by Ruth Chang (Harvard, 1999).


Inability to act reasonably because of weakness of will; lack of self-control.

Recommended Reading: Alfred R. Mele, Irrationality: An Essay on Akrasia, Self-Deception, and Self-Control (Oxford, 1992) and Robert Dunn, The Possibility of Weakness of Will (Hackett, 1987).

Also see David Carr.


Incapable of being corrected; hence, a putative criterion of certainty. An incorrigible proposition is one about which it is impossible to be mistaken, such as (perhaps) "I am now in pain." Whether any human knowledge is actually incorrigible is one of the central questions of epistemology.

Recommended Reading: Ludwig Wittgenstein, On Certainty (Harpercollins, 1986); Certainty, ed. by Jonathan Westphal (Hackett, 1995); and William P. Alston, Epistemic Justification: Essays in the Theory of Knowledge (Cornell, 1993).


An expression whose meaning depends upon the context in which it is employed. Thus, for example, in the sentence, "I came back from there an hour ago," the words "I" and "there," along with the phrase "an hour ago," are all indexicals—the person, place, and time to which they refer is different on each occasion of their use.

Recommended Reading: Philosophical Logic, ed. by T. J. Smiley (Oxford, 1999); John Perry, Reference and Reflexivity (CSLI, 2001); Lawrence D. Roberts, How Reference Works: Explanatory Models for Indexicals, Descriptions, and Opacity (SUNY, 1993); John Perry, The Problem of the Essential Indexical (CSLI, 2001); and Ingar Brinck, The Indexical 'I': The First Person in Thought and Language (Kluwer, 1997).

Also see SEP and John Perry.

indirect proof

Demonstration of the truth of a proposition from the impossible consequences of its contradictory; see reductio ad absurdum.

individual constant

A symbol (usually lowercase letters such as   a, b, c, etc.) used to represent a specific thing in quantification theory.

individual variable

A symbol (usually lowercase letters such as   x, y, z, etc.) used to represent any individual generally in quantification theory.


The characteristic of a proposition whose truth cannot be doubted, such as "My father is older than I am," even though (given bizarre suppositions about time and/or human conception) it might be false. Descartes and other modern philosophers supposed that only such propositions would provide a suitable foundation for human knowledge.

Recommended Reading: Michael Williams, Unnatural Doubts (Princeton, 1995); David Owens, Reason Without Freedom: The Problem of Epistemic Normativity (Routledge, 2000); and Nicholas Nathan, The Price of Doubt (Routledge, 2000).


Probable reasoning whose conclusion goes beyond what is formally contained in its premises; see deduction / induction.

Recommended Reading: Ian Hacking, An Introduction to Probability and Inductive Logic (Cambridge, 2001); Brian Skyrms, Choice and Chance: An Introduction to Inductive Logic (Wadsworth, 1999); D. P. Chattopadhyaya, Induction, Probability, and Skepticism (SUNY, 1991); Hilary Kornblith, Inductive Inference and Its Natural Ground: An Essay in Naturalistic Epistemology (MIT, 1993); and Elijah Milgram, Practical Induction (Harvard, 1999).

Also see SEP, DPM, EB, Rudy Garns, Richard Jeffrey, Soshichi Uchii, and CE.


Incapable of being expressed in language, as the experience of qualia generally and mystical insight in particular are sometimes held to be.

Recommended Reading: Ludwig Wittgenstein, Tractatus Logico-Philosophicus (Routledge, 1995) and Ben-Ami Scharfstein, Ineffability: The Failure of Words in Philosophy and Religion (SUNY, 1993).


The relationship that holds between the premises and the conclusion of a logical argument, or the process of drawing a conclusion from premises that support it deductively or inductively.

Recommended Reading: Henry E. Kyburg, Jr., Epistemology and Inference (Minnesota, 1982); D. S. Clarke, Jr., Practical Inferences (Routledge, 1985); Robert B. Brandom, Articulating Reasons: An Introduction to Inferentialism (Harvard, 2000); K.I. Manktelow, Inference and Understanding: A Philosophical and Psychological Perspective (Routledge, 1990); and Inference, Explanation, and Other Frustrations: Essays in the Philosophy of Science, ed. by John Earman (California, 1992).

Also see EB.

inference, rules of

Elementary valid argument forms whose substitution instances may be used to justify the steps in a formal proof of the validity of a more complex deductive argument. The rules of inference that we employ here include:

  1. Modus Ponens,
  2. Modus Tollens,
  3. Hypothetical Syllogism,
  4. Disjunctive Syllogism,
  5. Constructive Dilemma,
  6. Absorption,
  7. Simplification,
  8. Conjunction, and
  9. Addition.
These, taken together with suitable rules of replacement, adequately secure the completeness of the propositional calculus.

Also see Rudy Garns.

infinite regress

A definitional, explanatory, or justificatory procedure that entails its own reapplication without any limit. Thus, for example, the claim that everything in the world has only extrinsic value would lead to an infinite regress. Since the lack of any intrinsically worthwhile starting-point would render all value open to question, the procedure seems to be self-defeating.

Recommended Reading: John Passmore, Philosophical Reasoning (Basic, 1969).

informal fallacy

An attempt to persuade that obviously fails to demonstrate the truth of its conclusion, deriving its only plausibility from a misuse of ordinary language. The informal fallacies include:


Recommended Reading: Nicholas Capaldi, The Art of Deception: An Introduction to Critical Thinking (Prometheus, 1987); S. Morris Engel and Rudolf Steiner, With Good Reason: An Introduction to Informal Fallacies (Bedford, 1994); and Douglas N. Walton, Informal Fallacies: Towards a Theory of Argument Criticisms (Benjamins, 1987)

Also see Rudy Garns, SEP, and FF.

informative use of language

Communication employed for the purpose of asserting propositions or presenting arguments.

Ingarden, Roman (1893-1970)

Polish philosopher who developed a comprehensive aesthetic theory in Das literarische Kunstwerk (The Literary Work of Art: An Investigation on the Borderlines of Ontology, Logic, and Theory of Literature) (1931). Ingarden's Vom formalen Aufbau des individuellen Gegenstandes (The Structure of Individual Objects) (1935) proposed using the phenomenalist methods of Husserl to defend perceptual realism.

Recommended Reading: Bohdan Dziemidok, On the Aesthetics of Roman Ingarden: Interpretations and Assessments, ed. by Peter McCormick (Kluwer, 1989).

Also see SEP.


Latin term for natural capacity or understanding.

Also see PP.

innate ideas

Mental contents that are presumed to exist in the mind prior to and independently of any experience. Although rationalists like Plato and Descartes presume the existence of innate ideas, empiricists like Locke typically argue that there are no innate ideas. Noam Chomsky has proposed that grammatical structures, though not ideas, may be innate.

Recommended Reading: Innate Ideas, ed. by Stephen P. Stich (California, 1979); Peter Carruthers, Human Knowledge and Human Nature: A New Introduction to an Ancient Debate (Oxford, 1992); and Fiona Cowie, What's Within?: Nativism Reconsidered (Oxford, 1998).

Also see SEP on innatism and innateness in language, EB.


Belief that statements or theories may be used as tools for useful prediction without reference to their possible truth or falsity. Peirce and other pragmatists defended an instrumentalist account of modern science.

Recommended Reading: Charles S. Pierce: The Essential Writings, ed. by Edward C. Moore (Prometheus, 1998); Stephen Toulmin, Philosophy of Science (Harpercollins, 1985); Theo A. F. Kuipers, From Instrumentalism to Constructive Realism (Kluwer, 2000); and Michael Eldridge, Transforming Experience: John Dewey's Cultural Instrumentalism (Vanderbilt, 1998).

Also see EB.


The meaning of an expression. See extension / intension.


The characteristic feature of cognitive states—that they invariably represent or are about something beyond themselves. The intentions of a moral agent are, therefore, the states of mind that accompany its actions.

Recommended Reading: Daniel C. Dennett, The Intentional Stance (MIT, 1989); William Lyons, Approaches to Intentionality (Oxford, 1998); John R. Searle, Intentionality (Cambridge, 1983); Robert C. Stalnaker, Context and Content: Essays on Intentionality in Speech and Thought (Oxford, 1999); Hubert L. Dreyfus, Husserl, Intentionality and Cognitive Science (Bradford, 1990); Edward N. Zalta, Intentional Logic and the Metaphysics of Intentionality (MIT, 1988); and Michael Bratman, Faces of Intention: Selected Essays on Intention and Agency (Cambridge, 1999).

Also see SEP on intentionality, intentions, and intentionality in ancient philosophy, and intentionality and consciousness, DPM, John Perry, Pär Sundström, and David L. Thompson.


The supposition, defended by Descartes and others, that the minds and bodies of human beings exert direct causal influence on each other, even though they are distinct substances of different kinds.

Recommended Reading: The Incorporated Self, ed. by Michael O'Donovan-Anderson (Rowman & Littlefield, 1996); Karl Popper, Knowledge and the Body-Mind Problem: In Defence of Interaction (Routledge, 1996); and Matthew Buncombe, The Substance of Consciousness: An Argument for Interactionism (Avebury, 1995).

Also see EB.


The complex mosaic of relationships by means of which signifiers have meaning in the semantic theories of Lyotard and Kristeva.

Recommended Reading: Graham Allen, Intertextuality (Routledge, 2000); Criticism, History, and Intertextuality, ed. by Richard Fleming and Michael Payne (Bucknell, 1987); and Michael Worton and Judith Still, Intertextuality: Theories and Practices (Manchester, 1991).

intrinsic / extrinsic

Distinction between the features of things. The intrinsic features of a thing are those which it has in and of itself; while its extrinsic features are those which it has only in its relation to something else. Thus, for example, I am intrinsically a human being, but only extrinsically a father. It might reasonably be disputed whether my being male is an intrinsic biological feature or an extrinsic cultural construction.

In epistemology, the distinction between primary and secondary qualities points out the difference between the intrinsic and the extrinsic properties of material objects, and in normative ethics, deontologists and consequentialists disagree about whether the moral value of human actions resides in their intrinsic or their extrinsic features.

Recommended Reading: Noah M. Lemos, Intrinsic Value: Concept and Warrant (Cambridge, 1994) and Michael J. Zimmerman, The Nature of Intrinsic Value (Rowman & Littlefield, 2001).

Also see SEP.

intuition {Gk. νοησις [nóêsis]}

Direct, non-inferential awareness of abstract objects or concrete truths. Plato held that intuition is a superior faculty, and Spinoza supposed that intuition is the highest sort of human knowledge. Russell, on the other hand, designated as intuitive any unreflective instance of knowledge by acquaintance.

Recommended Reading: Victor Kal, On Intuition and Discursive Reasoning in Aristotle (Brill, 1988); Rethinking Intuition, ed. by Michael R. Depaul and William Ramsey (Rowman & Littlefield, 1999); David Weissman, Intuition and Ideality (SUNY, 1987); and Nel Noddings and Paul J. Shore, Awakening the Inner Eye: Intuition in Education (Educ. Int., 1998).

Also see EB and CE.


Reliance on unmediated awareness as a criterion of truth. In logic and mathematics, intuitionism denies the independent reality of mathematical objects and the principle of excluded middle. In moral philosophy, intuitionism is the metaethical theory that moral judgments are made by reference to a direct, non-inferential awareness of moral value. Ethical intuitionists usually hold that we recognize our duties in the specific features of particular moral decisions.

Recommended Reading: Gisele Fischer Servi, Intuitionism and Models of Cognition (Giro, 1996); Michael Dummett, Elements of Intuitionism (Oxford, 2000); Grant C. Sterling, Ethical Intuitionism and Its Critics (Peter Lang, 1994); and James Q. Wilson, Moral Intuitions (Transaction, 2000).

Also see J. R. Lucas, SEP, EB, and ISM.


Deductively ineffective; see valid / invalid.

Irigaray, Luce (1932- )

French psycholinguist and philosopher, author of Passions Elementaires (Elemental Passions) (1982), Ethique de la Difference Sexuelle (An Ethics of Sexual Difference) (1984), Je, Tu, Nous: Pour une Culture de la Difference (Je, Tu, Nous: Toward a Culture of Difference) (1990). Irigaray examines the systematic suppression of feminine and maternal concerns from the history of Western philosophy in Ce sexe qui n'en est pas un (This sex which is not one) (1977), arguing that valorization of the masculine is destructive to the fluid multiplicity of feminine sexuality. Her essays often try to convey the significance of subjectivity by modifying the conventions of putatively 'objective' speech. In Speculum de l'autre femme (Speculum of the Other Woman) (1974), Irigary argues that women can de-center the "master discourse" of linguistic communication by affirming their biological duality. Recent translations of Irigaray's work include Sexes and Geneologies (1993), To Be Two (2001), To Speak is Never Neuter (2001), and Democracy Begins Between Two (2001).

Recommended Reading: The Irigaray Reader, tr. and ed. by Margaret Whitford and David Macey (Blackwell, 1991); Joan Nordquist, French Feminist Theory: Luce Irigaray and Helene Cixous: A Bibliography (Ref. & Res., 1991); Margaret Whitford, Luce Irigaray: Philosophy in the Feminine (Routledge, 1991); Engaging with Irigaray, ed. by Carolyn Burke, Naomi Schor, and Margaret Whitford (Columbia, 1994); Tina Chanter, Ethics of Eros: Irigaray's Rewriting of the Philosophers (Routledge, 1995); and Tamsin E. Lorraine, Irigaray & Deleuze: Experiments in Visceral Philosophy (Cornell, 1999).

Also see FTW, EB, and ELC.


Use of language to convey something entirely different from its literal meaning. Thus, Socrates professed an ignorance that was the mark of true wisdom, and Kierkegaard often tried to provoke his readers by writing exactly the opposite of what he intended for them to believe.

Recommended Reading: Soren Kierkegaard, The Concept of Irony, tr. by Howard V. Hong (Princeton, 1992); Alexander Nehamas, The Art of Living: Socratic Reflections from Plato to Foucault (California, 2000); David Wisdo, The Life of Irony and the Ethics of Belief (SUNY, 1992); and Richard Rorty, Contingency, Irony and Solidarity (Cambridge, 1989).

Also see EB and Hugh Bredin.

irrelevant conclusion (ignoratio elenchi)

Reasoning that misses the point. The informal fallacy of defending the truth of a proposition by appeal to an argument that is actually concerned with something else.

Example: "Parents with large incomes can buy lots of things for their children. Therefore, the children of wealthy parents are happy."

Also see FF and GLF.


A general suffix commonly used to designate varieties of philosophical opinion. Although useful for some purposes, such labels should not be taken too seriously. Individual philosophers nearly always deal creatively with complex specific issues, developing arguments in defense of their own views. It is only later that we who read them find it convenient to invent simple names for positions that several of them seem to share. Study of philosophy benefits more from careful reading of the texts themselves than from the artificial classification of their themes.


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