The Collaborative International Dictionary of English v.0.48:

Think \Think\, v. t. [imp. & p. p. Thought; p. pr. & vb. n.
   Thinking.] [OE. thinken, properly, to seem, from AS.
   [thorn]yncean (cf. Methinks), but confounded with OE.
   thenken to think, fr. AS. [thorn]encean (imp.
   [thorn][=o]hte); akin to D. denken, dunken, OS. thenkian,
   thunkian, G. denken, d["u]nken, Icel. [thorn]ekkja to
   perceive, to know, [thorn]ykkja to seem, Goth. [thorn]agkjan,
   [thorn]aggkjan, to think, [thorn]ygkjan to think, to seem,
   OL. tongere to know. Cf. Thank, Thought.]
   1. To seem or appear; -- used chiefly in the expressions
      methinketh or methinks, and methought.
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   Note: These are genuine Anglo-Saxon expressions, equivalent
         to it seems to me, it seemed to me. In these
         expressions me is in the dative case.
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   2. To employ any of the intellectual powers except that of
      simple perception through the senses; to exercise the
      higher intellectual faculties.
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            For that I am
            I know, because I think.              --Dryden.
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   3. Specifically:
      (a) To call anything to mind; to remember; as, I would
          have sent the books, but I did not think of it.
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                Well thought upon; I have it here. --Shak.
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      (b) To reflect upon any subject; to muse; to meditate; to
          ponder; to consider; to deliberate.
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                And when he thought thereon, he wept. --Mark
                                                  xiv. 72.
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                He thought within himself, saying, What shall I
                do, because I have no room where to bestow my
                fruits?                           --Luke xii.
                                                  17.
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      (c) To form an opinion by reasoning; to judge; to
          conclude; to believe; as, I think it will rain
          to-morrow.
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                Let them marry to whom they think best. --Num.
                                                  xxxvi. 6.
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      (d) To purpose; to intend; to design; to mean.
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                I thought to promote thee unto great honor.
                                                  --Num. xxiv.
                                                  11.
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                Thou thought'st to help me.       --Shak.
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      (e) To presume; to venture.
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                Think not to say within yourselves, We have
                Abraham to our father.            --Matt. iii.
                                                  9.
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   Note: To think, in a philosophical use as yet somewhat
         limited, designates the higher intellectual acts, the
         acts preeminently rational; to judge; to compare; to
         reason. Thinking is employed by Hamilton as
         "comprehending all our collective energies." It is
         defined by Mansel as "the act of knowing or judging by
         means of concepts,"by Lotze as "the reaction of the
         mind on the material supplied by external influences."
         See Thought.
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   To think better of. See under Better.

   To think much of, or To think well of, to hold in esteem;
      to esteem highly.
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   Syn: To expect; guess; cogitate; reflect; ponder;
        contemplate; meditate; muse; imagine; suppose; believe.
        See Expect, Guess.
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The Collaborative International Dictionary of English v.0.48:

Thinking \Think"ing\, a.
   Having the faculty of thought; cogitative; capable of a
   regular train of ideas; as, man is a thinking being. --
   Think"ing*ly, adv.
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The Collaborative International Dictionary of English v.0.48:

Thinking \Think"ing\, n.
   The act of thinking; mode of thinking; imagination;
   cogitation; judgment.
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         I heard a bird so sing,
         Whose music, to my thinking, pleased the king. --Shak.
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WordNet (r) 3.0 (2006):

thinking
    adj 1: endowed with the capacity to reason [syn: intelligent,
           reasoning(a), thinking(a)]
    n 1: the process of using your mind to consider something
         carefully; "thinking always made him frown"; "she paused
         for thought" [syn: thinking, thought, thought
         process, cerebration, intellection, mentation]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "thinking":
   abstract thought, act of thought, assessment, assumption, attitude,
   belief, brainwork, cerebral, cerebration, climate of opinion,
   cogitation, cogitative, cognitive, common belief,
   community sentiment, conceit, concentrating, concentrative,
   concept, conception, conceptive, conceptual, conceptualization,
   conceptualized, conclusion, consensus gentium, consideration,
   contemplating, contemplative, creative thought, deliberating,
   deliberative, endopsychic, estimate, estimation, ethos, evaluation,
   excogitating, excogitation, eye, feeling, general belief, headwork,
   heavy thinking, idea, ideation, ideative, imageless thought,
   impression, intellection, intellectual, intellectual exercise,
   intellectualization, intelligent, internal, introspective,
   judgement, judgment, lights, meditating, meditative, mental,
   mental act, mental labor, mental process, mentation, mind, museful,
   musing, mystique, noesis, noetic, noological, notion, observation,
   opinion, outlook, pensive, personal judgment, philosophical,
   philosophy, phrenic, point of view, pondering, popular belief,
   position, posture, prehensive, presumption, prevailing belief,
   psychic, psychologic, public belief, public opinion, ratiocination,
   ratiocinative, rational, reaction, reasonable, reasoning,
   reflecting, reflective, ruminant, ruminating, ruminative, sensible,
   sentiment, serious, sight, sober, speculative, spiritual, stance,
   straight thinking, subjective, theory, thinking aloud,
   thinking out, thought, thoughtful, view, viewpoint,
   way of thinking, wistful