Source : Webster's Revised Unabridged Dictionary (1913)

      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.

Source : WordNet®

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