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With certain significant exceptions, every concept can be defined and communicated in terms of other concepts. The exceptions are concepts referring to sensations, and metaphysical axioms.

Sensations are the primary material of consciousness and, therefore, cannot be communicated by means of the material which is derived from them. The existential causes of sensations can be described and defined in conceptual terms (e.g., the wavelengths of light and the structure of the human eye, which produce the sensations of color), but one cannot communicate what color is like, to a person who is born blind. To define the meaning of the concept blue, for instance, one must point to some blue objects to signify, in effect: I mean this. Such an identification of a concept is known as an ostensive definition.

Ostensive definitions are usually regarded as applicable only to conceptualized sensations. But they are applicable to axioms as well. Since axiomatic concepts are identifications of irreducible primaries, the only way to define one is by means of an ostensive definitione.g., to define existence, one would have to sweep ones arm around and say: I mean this.

Definitions,
Introduction to Objectivist Epistemology, 4041

See also: Axiomatic Concepts; Axioms; Definitions; Irreducible Primaries; Perception; Self-Evident; Sensations.

.Copyright 1986 by Harry Binswanger. Introduction copyright 1986 by Leonard Peikoff. All rights reserved. For information address New American Library.

Acknowledgments

Excerpts from The Ominous Parallels, by Leonard Peikoff. Copyright 1982 by Leonard Peikoff. Reprinted with permission of Stein and Day Publishers. Excerpts from The Romantic Manifesto, by Ayn Rand. Copyright 1971, by The Objectivist. Reprinted with permission of Harper & Row, Publishers, Inc. Excerpts from Atlas Shrugged, copyright 1957 by Ayn Rand, The Fountainhead, copyright 1943 by Ayn Rand, and For the New Intellectual, copyright 1961 by Ayn Rand. Reprinted by permission of the Estate of Ayn Rand. Excerpts from Philosophy: Who Needs It, by Ayn Rand. Copyright 1982 by Leonard Peikoff, Executor, Estate of Ayn Rand. Reprinted by permission of the Estate of Ayn Rand. Excerpts from The Philosophy of Objectivism lecture series. Copyright 1976 by Leonard Peikoff. Reprinted by permission. Excerpts from Alvin Tofflers interview with Ayn Rand, which first appeared in Playboy magazine. Copyright 1964. Reprinted by permission of Alvin Toffler. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Plume, a member of Penguin Group (USA), Inc.