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"I" proposition

In the traditional notation for categorical logic, a proposition that is both particular and affirmative.

Example: "Some birds are Canada geese."

Such a proposition affirms that there is at least one thing that belongs to both of the designated classes. Its contradictory is an "E" proposition with the same subject and predicate terms.

Ibn Daud, Abraham ben David Hallevi (1110-1180)

Jewish philosopher. Ibn Daud was the first Jewish Aristotelean. His Sefer ha-Qabbalah (The Book of Tradition) (1161) and Emunah Ramah (The Exalted Faith) (1161) grounded Jewish theology on the metaphysics of Ibn Sina, providing an important influence on the work of Maimonides. Ibn Daud defended free will by proposing limitations on the extent of divine omnipotence.

Also see SEP and EB.

Ibn Gabirol, Solomon (1020-1057)
Ibn Gabirol

Jewish philosopher and poet. Translated into Latin as Fons Vitae (The Source of Life), Ibn Gabirol's philosophical work expressed a unique version of neoplatonism. His distinction between the essence and the will of god had significant influence on the thought of Duns Scotus.

Recommended Reading: Solomon B. Ibn-Gabirol, Improvement of the Moral Qualities: An Ethical Treatise of the Eleventh Century (AMS, 1990); Selected Poems of Solomon Ibn Gabirol, tr. by Peter Cole (Princeton, 2000); and Isaac Goldberg, Solomon Ibn Gabirol: A Bibliography (Word Works, 1998).

Also see SEP, EB, and CE.

Ibn Rushd, Abù al-Walìd Muhammad b. Ahmad (1126-1198)
Ibn Rushd

Andalusian Islamic philosopher who responded to the anti-philosophical tirades of al-Ghazàlì in Tahafut al tahafut (The Incoherence of the Incoherence) by defending the capacity of human reason to achieve knowledge independently of the neoplatonist doctrines of Ibn Sina and Maimonides. Ibn Rushd's exposition of the logical and metaphysical texts of Aristotle earned him the title of "The Commentator" among scholastic thinkers. He subjected theology to the claims of philosophy, holding that matter is eternal and allowing for immortality only as impersonal identification with the Agent Intellect shared by all. His arguments that knowledge is better founded on reason than on faith were greatly influential on Aquinas.

Recommended Reading: Oliver Leaman, Averroes and His Philosophy (Oxford, 1994); Averroes and the Enlightenment, ed. by Murad Wahbah and Mona Abousenna (Promethean, 1996); Barry Kogan, Averroes and the Metaphysics of Causation (SUNY, 1985); Roger Arnaldez, Averroes: A Rationalist in Islam, tr. by David Streight (Notre Dame, 2000); Majid Fakhry, Averroes, Aquinas, and the Rediscovery of Aristotle in Western Europe (Center for Muslim-Christian Understanding, 1997); and Dominique Urvoy, Ibn Rushd: Averroes, tr. by Olivia Stewart (Routledge, 1991).

Also see R. J. Kilcullen, EB, ELC, and CE.

Ibn Sina, Abù'Alì al-Husayn (980-1037)
Ibn Sina

Persian Islamic philosopher and physician whose Kitab Al-Shifa (Book of Healing) commented on the philosophy of Aristotle. As a leading neoplatonist, Ibn Sina emphasized the causal necessity that characterizes emanations from the divine, but supposed that human knowledge can best be achieved by mystical illumination.

Recommended Reading: Hakim G. M. Chishti, The Traditional Healer's Handbook: Classic Guide to the Medicine of Avicenna (Inner Traditions, 1992); Peter Heath, Allegory and Philosophy in Avicenna (Pennsylvania, 1992); Lenn E. Goodman, Avicenna (Routledge, 1992); Soheil M. Afnan, Avicenna, His Life and Works (Greenwood, 1980); Avicenna's Psychology, ed. by F. Rahman (Hyperion, 1981); and David B. Burrell, Knowing the Unknowable God: Ibn-Sina, Maimonides, Aquinas (Notre Dame, 1987).

Also see EB, ELC, and CE.

idea {Ger. Begriff}

The content of conscious thought. Plato used the Greek word ειδοι to designate the universal Forms. For modern representationalists like Descartes and Locke, however, ideas are the immediate objects of every mental activity. Ideas in this sense are supposed to represent things—present or absent—before the mind.

Recommended Reading: Gail Fine, On Ideas: Aristotle's Criticism of Plato's Theory of Forms (Clarendon, 1995); Margaret Dauler Wilson, Ideas and Mechanism (Princeton, 1999); David Hausman and Alan Hausman, Descartes's Legacy: Mind & Meaning in Early Modern Philosophy (Toronto, 1997); John W. Yolton, Locke and the Way of Ideas (St. Augustine, 1993); and Richard A. Watson, Representational Ideas: From Plato to Patricia Churchland (Kluwer, 1995).


Belief that only mental entities are real, so that physical things exist only in the sense that they are perceived. Berkeley defended his "immaterialism" on purely empiricist grounds, while Kant and Fichte arrived at theirs by transcendental arguments. German, English, and (to a lesser degree) American philosophy during the nineteenth century was dominated by the monistic absolute idealism of Hegel, Bradley, and Royce.EB,

Recommended Reading: David Berman, George Berkeley: Idealism and the Man (Oxford, 1996); German Idealist Philosophy, ed. by Rudiger Bubner (Penguin, 1997); The Cambridge Companion to German Idealism, ed. by Karl Ameriks (Cambridge, 2001); John Foster, The Case for Idealism (Routledge, 1982); and Current Issues in Idealism, ed. by Paul Coates and Daniel D. Hutto (St. Augustine, 1997).

Also see IEP, EB, ISM, DPM, and CE.


The logical relation of numerical sameness, in which each thing stands only to itself. Although everything is what it is and not anything else, philosophers try to formulate more precisely the criteria by means of which we may be sure that one and the same thing is cognized under two different descriptions or at two distinct times. Leibniz held that numerical identity is equivalent to indiscernibility or sameness of all the features each thing has. But Locke maintained that judgments of identity are invariably made by reference to types or sorts of things. The identity of individual persons is an especially troublesome case.

Recommended Reading: Colin McGinn, Logical Properties: Identity, Existence, Predication, Necessity, Truth (Clarendon, 2001); Eli Hirsch, The Concept of Identity (Oxford, 1992); Andre Gallois, Occasions of Identity: A Study in the Metaphysics of Persistence, Change, and Sameness (Clarendon, 1998); Katherine Hawley, How Things Persist (Oxford, 2001); David Wiggins, Sameness and Substance Renewed (Cambridge, 2001); Individuation and Identity in Early Modern Philosophy, ed. by Kenneth F. Barber and Jorge J.E. Garcia (SUNY, 1994); Self and Identity: Contemporary Philosophical Issues, ed. by Daniel Kolak and Raymond Martin (Prentice Hall, 1991); and Particulars, Actuality, and Identity Over Time, ed. by Michael Tooley (Garland, 1999).

Also see William J. Greenberg, SEP on identity, identity through time, transworld identity, relative identity and the identity of indiscernibles, EB, and Loretta Torrago.

identity theory of mind

Belief that mental properties and events are identical with physical properties and events. Although the details are not yet apparent, identity theorists suppose that scientific research into the nature of the central nervous system will eventually establish the contingent identity of every kind of conscious experience with some neurophysiological phenomenon. Significant variations of the identity theory include physicalism and neutral monism.

Recommended Reading: Cynthia MacDonald, Mind-Body Identity Theories (Routledge, 1990); D. M. Armstrong, A Materialist Theory of the Mind (Routledge, 1993); Patricia Smith Churchland, Neurophilosophy: Toward a Unified Science of the Mind-Brain (MIT, 1986); Hilary Putnam, Representation and Reality (MIT, 1991); and J. Allan Hobson, States of Brain and Mind (Birkhauser, 1988).

Also see SEP, IEP, EB, and DPM.

ignorance, appeal to (argumentum ad ignoratiam)

The informal fallacy of supposing that a proposition must be true because there is no proof that it is false.

Example: "The F.B.I. investigation was never able to establish that Smith was not at the scene of the crime on the night of June 25th, so we may safely conclude that he was there."

Recommended Reading: Douglas N. Walton, Arguments from Ignorance (Penn. State, 1996).

Also see FF and GLF.

ignoratiam, argumentum ad

See appeal to ignorance.

ignoratio elenchi

Latin phrase meaning, "misunderstanding of the refutation." See irrelevant conclusion.

illicit major

The formal fallacy committed in a categorical syllogism that is invalid because its major term is undistributed in the major premise but distributed in the conclusion.

Example: "All dogs are mammals. No cats are dogs. Therefore, no cats are mammals."

Also see FF and GLF.

illicit minor

The formal fallacy committed in a categorical syllogism that is invalid because its minor term is undistributed in the minor premise but distributed in the conclusion.

Example: "All poodles are mammals. All poodles are pets. Therefore, All pets are mammals."

Also see FF and GLF.

illocutionary act

The speech act of doing something else—offering advice or taking a vow, for example—in the process of uttering meaningful language. Thus, for example, in saying "I will repay you this money next week," one typically performs the illocutionary act of making a promise.

Recommended Reading: How to Do Things With Words (Harvard, 1975) J. L. Austin, ; John R. Searle, Speech Acts (Cambridge, 1970); William P. Alston, Illocutionary Acts and Sentence Meaning (Cornell, 2000); and Jerrold J. Katz, Propositional Structure and Illocutionary Force: A Study of the Contribution of Sentence Meaning to Speech Acts (Harvard, 1980).

imagination {Gk. εικασια [eikásia]}

The capacity to consider sensible objects without actually perceiving them or supposing that they really exist. Philosophers have disagreed over whether or not acts of imagination necessarily involve mental images or ideas.

Recommended Reading: Mary Warnock, Imagination and Time (Blackwell, 1994); Edward S. Casey, Imagining: A Phenomenological Study (Indiana, 2000); Mark Johnson, The Body in the Mind: The Bodily Basis of Meaning, Imagination, and Reason (Chicago, 1990); and Roger Scruton, Art and Imagination: A Study in the Philosophy of Mind (St. Augustine, 1997).

Also see SEP on imagination and mental imagery.

immediate inference

The relationship between two propositions that are logically equivalent. In categorical logic, the traditional immediate inferences include: conversion, obversion, and contraposition.


The absence of any bias toward or away from a particular person or opinion. Enlightenment philosophers often upheld the use of human reason as an impartial tool, but postmodern thinkers raise significant doubts about the possibility and value of such objectivity. Although moral impartiality has traditionally been regarded as a virtue, in strict practice it would require callous disregard for every special relationship with another person. In public life, however, impartiality is a crucial component of justice.

Recommended Reading: Stephen L. Darwall, Impartial Reason (Cornell, 1995); Shane O'Neill, Impartiality in Context: Grounding Justice in a Pluralist World (SUNY, 1997); and Paul Kelly, Impartiality, Neutrality and Justice (Columbia, 2001).


Relation between two propositions, one of which may be inferred from the other.
 p  q  p

A material implication is a compound statement that is true except when its first component statement (the antecedent is true and its second (the consequent) is false. Thus, the truth of the antecedent ensures that of the consequent. Material implication is symbolized here in the form:

	p ⊃ q

Example: "If Bob is competent, then Bob should get the job."

A strict implication (or entailment) is a tautologous statement of the same form.

Example: "If George is the same height as Janet, then Janet is the same height as George."

Recommended Reading: David H. Sanford, If P, Then Q: Conditionals and the Foundations of Reasoning (Routledge, 1992); W. L. Harper, R. Stalnaker, and G. Pearce, Ifs: Conditionals, Belief, Decision, Chance, and Time (Kluwer, 1980); and Michael Woods, Conditionals, ed. by David Wiggins and Dorothy Edgington (Clarendon, 1997).

Implication (Impl.)

A rule of replacement of the form:

	( p ⊃ q ) ≡  ( ~ p ∨ q )

Example: "If it rains, then we cancel the picnic." is equivalent to "Either it doesn't rain or we cancel the picnic."

The truth-table analysis of this equivalence amounts to a definition of material implication in terms of negation and disjunction.


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