The Collaborative International Dictionary of English v.0.48:

Principle \Prin"ci*ple\, n. [F. principe, L. principium
   beginning, foundation, fr. princeps, -cipis. See Prince.]
   1. Beginning; commencement. [Obs.]
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            Doubting sad end of principle unsound. --Spenser.
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   2. A source, or origin; that from which anything proceeds;
      fundamental substance or energy; primordial substance;
      ultimate element, or cause.
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            The soul of man is an active principle. --Tillotson.
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   3. An original faculty or endowment.
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            Nature in your principles hath set [benignity].
                                                  --Chaucer.
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            Those active principles whose direct and ultimate
            object is the communication either of enjoyment or
            suffering.                            --Stewart.
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   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.
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            Therefore, leaving the principles of the doctrine of
            Christ, let us go on unto perfection. --Heb. vi. 1.
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            A good principle, not rightly understood, may prove
            as hurtful as a bad.                  --Milton.
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   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.
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            All kinds of dishonesty destroy our pretenses to an
            honest principle of mind.             --Law.
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   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.
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            Cathartine is the bitter, purgative principle of
            senna.                                --Gregory.
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   Bitter principle, Principle of contradiction, etc. See
      under Bitter, Contradiction, etc.
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WordNet (r) 3.0 (2006):

bitter principle
    n 1: any one of several hundred compounds having a bitter taste;
         not admitting of chemical classification