Prin·ci·ple n.
 1. Beginning; commencement. [Obs.]
    Doubting sad end of principle unsound.   --Spenser.
 2. A source, or origin; that from which anything proceeds; fundamental substance or energy; primordial substance; ultimate element, or cause.
    The soul of man is an active principle.   --Tillotson.
 3. An original faculty or endowment.
    Nature in your principles hath set [benignity].   --Chaucer.
    Those active principles whose direct and ultimate object is the communication either of enjoyment or suffering.   --Stewart.
 4. A fundamental truth; a comprehensive law or doctrine, from which others are derived, or on which others are founded; a general truth; an elementary proposition; a maxim; an axiom; a postulate.
    Therefore, leaving the principles of the doctrine of Christ, let us go on unto perfection.   --Heb. vi. 1.
    A good principle, not rightly understood, may prove as hurtful as a bad.   --Milton.
 5. A settled rule of action; a governing law of conduct; an opinion or belief which exercises a directing influence on the life and behavior; a rule (usually, a right rule) of conduct consistently directing one's actions; as, a person of no principle.
    All kinds of dishonesty destroy our pretenses to an honest principle of mind.   --Law.
 6. Chem. Any original inherent constituent which characterizes a substance, or gives it its essential properties, and which can usually be separated by analysis; -- applied especially to drugs, plant extracts, etc.
    Cathartine is the bitter, purgative principle of senna.   --Gregory.
 Bitter principle, Principle of contradiction, etc. See under Bitter, Contradiction, etc.
 Con·tra·dic·tion n.
 1. An assertion of the contrary to what has been said or affirmed; denial of the truth of a statement or assertion; contrary declaration; gainsaying.
 His fair demands
 Shall be accomplished without contradiction.   --Shak.
 2. Direct opposition or repugnancy; inconsistency; incongruity or contrariety; one who, or that which, is inconsistent.
 can he make deathless death? That were to make
 Strange contradiction.   --Milton.
    We state our experience and then we come to a manly resolution of acting in contradiction to it.   --Burke.
    Both parts of a contradiction can not possibly be true.   --Hobbes.
    Of contradictions infinite the slave.   --Wordsworth.
 Principle of contradiction Logic, the axiom or law of thought that a thing cannot be and not be at the same time, or a thing must either be or not be, or the same attribute can not at the same time be affirmed and and denied of the same subject; also called the law of the excluded middle.
 Note: It develops itself in three specific forms which have been called the “Three Logical Axioms.” First, “A is A.” Second, “A is not Not-A” Third, “Everything is either A or Not-A.”
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 Ex·clude v. t. [imp. & p. p. Excluded; p. pr. & vb. n. Excluding.]
 1. To shut out; to hinder from entrance or admission; to debar from participation or enjoyment; to deprive of; to except; -- the opposite to admit; as, to exclude a crowd from a room or house; to exclude the light; to exclude one nation from the ports of another; to exclude a taxpayer from the privilege of voting.
    And none but such, from mercy I exclude.   --Milton.
 2. To thrust out or eject; to expel; as, to exclude young animals from the womb or from eggs.
 Excluded middle. logic The name given to the third of the  “three logical axioms,” so-called, namely, to that one which is expressed by the formula: “Everything is either A or Not-A.” no third state or condition being involved or allowed. See Principle of contradiction, under Contradiction.
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 Im·pos·si·bil·i·ty n.; pl. Impossibilities
 1. The quality of being impossible; impracticability.
    They confound difficulty with impossibility.   --South.
 2. An impossible thing; that which is not possible; that which can not be thought, done, or endured.
    Impossibilities! O, no, there's none.   --Cowley.
 3. Inability; helplessness. [R.]
 Logical impossibility, a condition or statement involving contradiction or absurdity; as, that a thing can be and not be at the same time. See Principle of Contradiction, under Contradiction.
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