thinker

1. One who devotes much time to thought or meditation.
2. One who thinks or reasons in a certain way: a careful thinker.

noun

1. A person who seeks reason and truth by thinking and meditation: philosopher. See thoughts.
2. A person of great mental ability: brain, intellect, intellectual, mind. See ability/inability.

Definition: theorist
Antonyms: doer, person of action

The noun has 2 meanings:

Meaning #1: an intellectual being
  Synonyms: mind, creative thinker

Meaning #2: someone who exercises the mind (usually in an effort to reach a decision)