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"O" proposition

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

Example: "Some trees are not evergreens."

Such a proposition affirms that there is at least one tree that is not also an evergreen. Its contradictory is an "A" proposition with the same subject and predicate terms.

objective / subjective

Distinction between propositions or judgments about the way things are and those about how people think or feel about them. The truth of objective claims is presumed to be entirely independent of the merely personal concerns reflected in subjective expressions, even though is difficult to draw the distinction precisely. Thus, for example: "Spinach is green" is objective, while "I like spinach" is subjective. "Seventy-three percent of people in Houston don't like spinach," however, seems to be an objective claim about certain subjects.

The legitimacy of this distinction is open to serious question, since it is unclear whether (and how) any knowing subject can achieve genuine objectivity. Nevertheless, because objective truth is supposed to carry undeniable persuasive force, exaggerated claims of objectivity have often been used as tools of intellectual and social oppression.

Recommended Reading: Richard Rorty, Objectivity, Relativism, and Truth (Cambridge, 1991); Thomas Nagel, The View from Nowhere (Oxford, 1989); Gábor Forrai, Reference, Truth and Conceptual Schemes: A Defense of Internal Realism (Kluwer, 2001); Bas C. Van Fraassen, The Empirical Stance (Yale, 2002); Richard J. Bernstein, Beyond Objectivism and Relativism: Science, Hermeneutics, and Praxis (Pennsylvania, 1983); and The Authority of Reason, ed. by Jean E. Hampton and Richard A. Healey (Cambridge, 1998).

Also see IEP.


That which binds us to act in accordance with duty. Obligatory acts are those one is morally, legally, or contractually required to perform. Normative theories usually try to explain the grounds of moral obligation as well as its practical application.

Recommended Reading: Michael J. Zimmerman, The Concept of Moral Obligation (Cambridge, 1996).

Also see SEP.

obscurum per obscurius

Latin phrase meaning "the obscure through the more obscure;" hence, the flaw in any account that proves more difficult to understand than what it purports to clarify. In this vein, philosophers reflecting upon each other's work sometimes echo Byron's complaint about Coleridge:

"But, like a hawk encumbered with his hood,
Explaining metaphysics to the nation—
I wish he would explain his explanation."


The reciprocal relationship between two categorical propositions of opposite quality such that the predicate term of each is the complement of the predicate term of the other. Obversion is a valid immediate inference for categorical propositions of every form. Thus, for example:

All lizards are reptiles and No lizards are non-reptiles

No spiders are insects and All spiders are non-insects

Some fish are birds and Some fish are not non-birds

Some mammals are not dogs and Some mammals are non-dogs

are all legitimate cases of obversion; either member of each pair can be substituted for the other.


Belief that natural events are not directly related in causation, since both the apparent cause and the apparent effect are, in fact, produced by some third thing (usually divine providence). Geulincx and Malebranche introduced occasionalism as an improved way of reconciling the mechanism with the dualism of Descartes.

Recommended Reading: Nicolas Malebranche, The Search After Truth, ed. by Thomas M. Lennon and Paul J. Olscamp (Cambridge, 1997) and The Cambridge Companion to Malebranche, ed. by Steven M. Nadler (Cambridge, 2000).

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

Ockham, William of (1285-1349)

English philosopher who defended the logic, physics, and metaphysics of Aristotle in Summa Logicae (The Whole of Logic) (1328) vol. 1 and vol. 2 and the Dialogus. An extreme nominalist, Ockham held that general terms are signs that indefinitely signify discrete (though similar) particulars. Ockham is best known for his statement of the law of parsimony as the ontological principle often called Ockham's Razor: "Frustra fit per plura quod potest fieri per pauciora" ["It is pointless to do with more what can be done with less"]. Thus, according to Ockham, we ought never to postulate the reality of any entity unless it is logically necessary to do so.

Recommended Reading: William of Ockham, Opera Philosophica (Franciscan, 1975); William of Ockham, Philosophical Writings: A Selection, tr. by Philotheus Boehner (Hackett, 1990); The Cambridge Companion to Ockham, ed. by Paul Vincent Spade (Cambridge, 1999); Rega Wood, Ockham on the Virtues (Purdue, 1997); and Five Texts on the Mediaeval Problem of Universals: Porphyry, Boethius, Abelard, Duns Scotus, Ockham, ed. by Paul Vincent Spade (Hackett, 1994).

Also see SEP, IEP, R. J. Kilcullen, EB, Yiwei Zheng, CE, MMT, and ELC.


Literally, rule by the few. Both Plato and Aristotle assumed that the small group most likely to gain control over the governance of a city-state would be those with great wealth.

Recommended Reading: Plato, The Republic, tr. by Desmond Lee (Viking, 1979) and Aristotle, Politics (Oxford, 1998).

Also see EB.

ontological argument

An attempt to prove the existence of god by a priori reasoning from the content of the concept of god. As formulated by Anselm, the ontological argument begins with a notion of "that than which nothing greater can be conceived." Anything that satisfies this concept must exist in reality as well as in thought (since otherwise it would be possible to conceive something greater—one that really exists); hence, god exists.

Descartes endorsed a different version of this argument, and Spinoza also relied upon it, but Kant rejected it because of the unintelligibility of comparing the relative greatness of real and merely possible beings. A form of the argument that emphasizes god's possession of the attribute of necessary existence has been defended in recent decades.

Recommended Reading: Anselm of Canterbury: The Major Works, ed. by Brian Davies and G. R. Evans (Oxford, 1998); Alvin Plantinga, God, Freedom, and Evil (Eerdmans, 1978); and The Ontological Argument, from St. Anselm to Contemporary Philosophers, ed. by Alvin Plantinga (Anchor, 1989).

Also see SEP on ontological arguments and Descartes' ontological argument, EB, Maria de Lourdes Borges, and J. R. Lucas.


Branch of metaphysics concerned with identifying, in the most general terms, the kinds of things that actually exist. Thus, the "ontological commitments" of a philosophical position include both its explicit assertions and its implicit presuppositions about the existence of entities, substances, or beings of particular kinds.

Recommended Reading: Jean-Paul Sartre, Being and Nothingness: A Phenomenological Essay on Ontology, tr. by Hazel E. Barnes (Washington Square, 1993); Herman Philipse, Heidegger's Philosophy of Being (Princeton, 1998); Ian Hacking, Historical Ontology (Harvard, 2002); Gustav Bergmann, New Foundations of Ontology, ed. by William Heald and Edwin B. Allaire (Wisconsin, 1992); W. V. O. Quine, Ontological Relativity (Columbia, 1977); and Roger F. Gibson, Jr., The Philosophy of W. V. Quine: An Expository Essay (Florida, 1986).

Also see DPM, EB, and CE.

open question argument

G. E. Moore's contention that any effort to define the good must fail, since it always remains possible to ask significantly whether or not the proposed definiens is actually good.

Recommended Reading: G. E. Moore, Principia Ethica (Prometheus, 1988); Brian Hutchinson, G. E. Moore's Ethical Theory: A Reassessment (Cambridge, 2001); and Tom Regan, Bloomsbury's Prophet: G. E. Moore and the Development of His Moral Philosophy (Temple, 1986).

Also see IEP, and R. N. Johnson.


Belief that the meaning of scientific terms and concepts is wholly captured by a description of the process that determines their applicability in particular cases. On this view, theoretical entities are merely logical constructs.

Recommended Reading: Charles S. Pierce: The Essential Writings, ed. by Edward C. Moore and Richard Robin (Prometheus, 1998) and Percy W. Bridgman, Logic of Modern Physics (Ayer, 1980).

Also see SEP and EB.

opinion {Gk. δοξα [dóxa]; Lat. sententia}

Acceptance of a proposition despite a lack of the conclusive evidence that would result in certain knowledge of its truth.


Belief that everything happens for the best {Lat. optimus}; the opposite of pessimism. Thus, for example, Leibniz was an optimist who supposed that this is the best of all possible worlds.

Recommended Reading: Gottfried Wilhelm Leibniz, Theodicy: Essays on the Goodness of God, the Freedom of Man, and the Origin of Evil, ed. by Austin Marsden Farrer (Open Court, 1988) and The Cambridge Companion to Leibniz, ed. by Nicholas Jolley (Cambridge, 1994).

Also see ISM and PP.

Ortega y Gasset, José (1883-1955)
Ortega y Gasset

Spanish politician and philosopher who taught at Madrid; author of La deshumanización del arte e ideas sobre la novela (The Dehumanization of Art) (1923), Misión de la Universidad (The Mission of the University) (1930), En torno a Galileo (Man and Crisis) (1933), ¿Qué est filosofía? (What is Philosophy?) (1934), and Historia como sistema (History as a System) (1941). Ortega's thought emphasized the unique reality and perspectival independence of each unique person in Meditaciones del Quijote (Meditations on Quixote) (1914) : "yo soy yo y mi circumstancia" ["I am myself and my circumstances"]. His political concerns, best expressed in La rebelión de las masas (The Revolt of the Masses) (1930), resulted in Ortega's exile from Spain during its Civil War.

Recommended Reading: Carlos Ramos Mattei, Ethical Self-Determination in Don Jose Ortega Y Gasset (Peter Lang, 1988); Oliver W. Holmes, Human Reality and the Social World: Ortega's Philosophy of History (Massachusetts, 1976); and Antonio Rodriguez Huescar, Jose Ortega Y Gasset's Metaphysical Innovation: A Critique and Overcoming of Idealism, ed. by Jorge Garcia-Gomez (SUNY, 1995).

Also see SEP, EB and ELC.


The attempt to provide a non-linguistic definition of a term by pointing at something to which it applies. Although useful enough for some primitive purposes, ostensive definitions are systematically ambiguous, since they poorly discriminate among things and their temporal features.

Recommended Reading: Ludwig Wittgenstein, Blue and Brown Books (HarperCollins, 1986); Routledge Philosophy Guidebook to Wittgenstein and the Philosophical Investigations, ed. by Marie McGinn (Routledge, 1997); Ludwig Wittgenstein: Philosophy and Language, ed. by Alice Ambrose and Morris Lazerowitz (St. Augustine, 1996); and William H. Brenner, Wittgenstein's Philosophical Investigations (SUNY, 1999).

Otto, Rudolf (1869-1937)

German philosopher of religion. Extrapolating from the philosophy of Kant in Naturalistische und religiöse Weltansich (Naturalism and Religion) (1904) and Das Heilige (The Idea of the Holy) (1917), Otto tried to identify, describe, and classify the variety of "numinous" feelings that give rise to the cognitive content of religious belief.

Recommended Reading: Rudolf Otto, Autobiographical and Social Essays, ed. by Gregory D. Alles (De Gruyter, 1996); Philip C. Almond, Rudolf Otto: An Introduction to His Philosophical Theology (North Carolina, 1992); Melissa Raphael, Rudolf Otto and the Concept of Holiness (Clarendon, 1997); and Todd A. Gooch, The Numinous and Modernity: An Interpretation of Rudolf Otto's Philosophy of Religion (De Gruyter, 2000).

Also see Kelly L. Ross, EB, ELC, and Stephen Palmquist.

ουσια [ousia]

Greek term for being or substance. Thus, in the philosophy of Aristotle, the most crucial of the categories by means of which to describe a natural object.

Recommended Reading: F. E. Peters, Greek Philosophical Terms: A Historical Lexicon (NYU, 1967).

Also see Christopher P. Long and PP.


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