Synonyms for Thinking:

adj.

deep in thought (adjective)
contemplative.
intellectual (adjective)
brainy, cerebral, egghead, intellectual, intelligent, psychological, rational, reasoning, understanding.
pertaining to thought (adjective)
cogitative, contemplative, engrossed, introspective, pensive, rational, reasonable, reflective, studious, Deliberating, Meditating.
rational (adjective)
intelligent, reasoning.
reasonable (adjective)
cogent, conceivable, deductive, feasible, inductive, likely, logical, plausible, reasonable, sensible, sound, wise, thought-out, intelligent, rational.
ruminative (adjective)
ruminative, in a brown study.
thoughtful (adjective)
absorbed, deliberative, meditative, ruminative, Ruminating.

n.

alert, clear, clearly, lucid, sane, well-adjusted, wide-awake, Compos-mentis, with-it. approach, cogitative, contemplative, deliberative, excogitative, framework, meditative, mindset, pensive, perspective, pragmatism, reflective, ruminative, school of thought, speculative, thoughtful, thoughts, in a brown study. attitude, feeling, impression, reputation, stance, stand, viewpoint, self-assessment. assessment, focus, forethought, introspection, soul-searching. imagination (noun)
brainchild, conception, creation, creativity, dream, fantasy, idea, invention, supposition, vision, Daydreams, Dreams.
intellect (noun)
brain, capacity, comprehension, intellect, intelligence, mentality, mind, rationality, understanding, brain-power.
memory (noun)
mind.
rational (noun)
absorbed, contemplative, engrossed, intent on, introspective, meditative, pensive, reflective, speculative, studious, thoughtful, Ruminating.
reason (noun)
conceptualization, deduction, induction, inference, logic, reason, synthesis.
thought (noun)
absorption, analysis, appraisal, attention, brooding, calculation, cerebration, cogitation, concentration, consideration, contemplation, daydreaming, deliberation, engrossment, envisioning, evaluation, idea, imagination, intellection, introspection, logic, lucubration, meditation, musing, pondering, preoccupation, reason, reasoning, reflection, speculation, study, thought, understanding, weighing, wistfulness, Debating, brain work.

v.

calculating (verb)
accounting, appraising, calculating, computing, concluding, counting, determining, dividing, figuring, guessing, judging, measuring, planning, programming, ranking, rating, reckoning, scheduling, scheming, scoring, studying, Adding, Approximating, Assessing, Deducing, Enumerating, Estimating, Gauging, Inferring, Multiplying, Plotting, Tallying, Triangulating, Valuing, Totaling, Considering, Presuming, Summing, Supposing, Surmising, Systematizing, rationalizing, evaluating, quantifying, quantizing.
imagining (verb)
brainstorming, designing, observing, Inventing, Opining, Perceiving, Theorizing, Conceiving, Creating, fantasizing.
reasoning (verb)
Analyzing, Cogitating, Inducting, conceptualizing, synthesizing.
thinking (verb)
reflecting, Deliberating, Imagining, Meditating, Concentrating, Contemplating, Speculating, cerebrating.

Other synonyms:

deliberative, excogitative, lucid, ruminative, well-adjusted, Compos-mentis. clearly, contemplative, meditative, pensive, reflective, sane. alert, speculative. clear. Other relevant words:
assessment, attitude, clear, clearly, cogitative, contemplative, deliberative, engrossed, excogitative, feeling, impression, introspective, lucid, meditative, pensive, reflective, ruminative, sane, speculative, stance, studious, thoughtful, viewpoint, Ruminating, in a brown study.

Usage examples for thinking

  1. I didn't sleep last night thinking of it. – Northern Lights by Gilbert Parker
  2. Why are you thinking of doing that? – Boy Scouts in Mexico; or On Guard with Uncle Sam by G. Harvey Ralphson
  3. She was thinking of- him? – The Black Bag by Louis Joseph Vance