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Q.E.D. (quod erat demonstrandum)

Latin for "what was to be proven." Hence, a common way of identifying the conclusion of a mathematical or logical argument. (It doesn't really mean "Quite Easily Done.")


The intrinsic phenomenal features of subjective consciousness, or sense data. Thus, qualia include what it is like to see green grass, to taste salt, to hear birds sing, to have a headache, to feel pain, etc. Providing an adequate account of qualia is sometimes held to be a difficult problem for functionalist explanations of mental states.

Recommended Reading: Leopold Stubenberg, Consciousness and Qualia (Benjamins, 1998); Emotion, Qualia and Consciousness, ed. by Alfred Kaszniak (World Scientific, 2001); Ming Singer, Unbounded Consciousness: Qualia, Mind and Self (Free Assn., 2001); and Joseph Levine, Purple Haze: The Puzzle of Consciousness (Oxford, 2000).

Also see SEP, DPM, SEP, Daniel Dennett, David J. Chalmers, and Eric Lormand.


The properties or features of things, whether they are intrinsic or extrinsic to the thing itself. See primary / secondary qualities.

Recommended Reading: Peter Alexander, Ideas, Qualities and Corpuscles: Locke and Boyle on the External World (Cambridge, 1983); Nelson Goodman, A Study of Qualities (Garland, 1993); and Austen Clark, Sensory Qualities (Oxford, 1996).

quality (propositional)

Along with propositional quantity, one of the distinguishing features among categorical propositions:

an affirmative proposition (A or I) states a relation of inclusion between members of the classes designated by its terms;
a negative proposition (E or O) states a relation of exclusion between members of the classes designated by its terms.

quantification rules

Valid argument forms, including

  1. Universal Instantiation,
  2. Universal Generalization,
  3. Existential Instantiation, and
  4. Existential Generalization,
whose substitution instances may be used to manipulate the use of quantifiers in a formal proof of the validity of a more complex deductive argument.

quantification theory

The formal system of logic (also known as the predicate calculus) that incorporates the entire propositional calculus and adds a set of quantification rules.


Logical symbols used in the predicate calculus (or quantification theory) to indicate the extent of the application of a propositional function.

The universal quantifier, (x) , indicates that the proposition applies to everything, so that "(x)( Fx ⊃ Gx )" may be read as, "For any x, if x is F then x is G."

The existential quantifier, (∃x) , indicates that the proposition applies to something, so that "(∃x)( Fx • Gx )" may be read as, "There is at least one x such that x is F and x is G."

Recommended Reading: The Frege Reader, ed. by Michael Beaney (Blackwell, 1997) and Alex Orenstein, Existence and the Particular Quantifier (Temple, 1979).

quantity (propositional)

Along with propositional quality, one of the distinguishing features among categorical propositions:

a universal proposition (A or E) refers to all members of the class designated by its subject term;
a particular proposition (I or O) refers only to some members of that class.

quantum mechanics

A physical theory developed by Planck, Heisenberg, and Schrödinger. Quantum theory typically permits only probable or statistical calculation of the observed features of subatomic particles, understood in terms of wave functions.

Recommended Reading: Roland Omnes, Understanding Quantum Mechanics (Princeton, 1999); Peter Kosso, Appearance and Reality: An Introduction to the Philosophy of Physics (Oxford, 1997); Hans Reichenbach, Philosophic Foundations of Quantum Mechanics (Dover, 1998); A. P. French and Edwin F. Taylor, Introduction to Quantum Physics (Norton, 1978); What Is Quantum Mechanics?: A Physics Adventure, ed. by John Nambu and Philip Heit (Blackwell, 1996); and Christopher Norris, Quantum Theory and the Flight from Realism (Routledge, 2000).

Also see EB, SEP on quantum mechanics, uncertainty, the Copenhagen interpretation, the many worlds interpretation, the relational interpretation, the modal interpretation, Everett's formulation, decoherence, quantum entanglement, the equivalence of mass and energy, quantum logic and probability theory, measurement and identity and individuality in quantum theory, and experiment in physics.

quaternio terminorum

Latin name for the fallacy of four terms.

Quine, Willard Van Orman (1908-2000)

American philosopher. Quine employs the methods of pragmatism and logical positivism in the support of a strict linguistic nominalism. He argued that neither the analytic/synthetic distinction nor the equivalence of meaning are defensible on strictly empiricist grounds.

For a discussion of his life and work, see Quine.


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Last modified 30 December 2011.
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