In terms, thinker (LH:3), the noun of the verb "think", is one who devotes considerable time to working and exercising their brain, which can be measured in terms of "powerfulness" (Einstein, c.1925), depending upon the successfullness of the thinking; one who devotes considerable time to thinking, thought, problem solving, ideas, theories, or ruminations, etc..
There are different types of thinkers, or rather "thinking states", which can be discerned depending on the hand, arm, and body posture, in respect to contact with head or brain; each posture mediating different brain wave patterns, or electromagnetic circuit flows, so to say, correlative to the thoughts or states of mind in context, such as shown below:
The finger on mouth, above left, indicate an "idea" stage of thinking. The three in middle showing a variety of thinking types. The following statement by Andrei Sakharov, the father of the Soviet hydrogen bomb, pictured above right thinking, seems to indicate a highly-pressurized type of thinking:
- “I worked under conditions of the greatest pressure, first in Moscow and subsequently in a special secret research center. We were fascinated by the grandeur of the task.”
- — Andrei Sakharov (c.1970), Publication
Generally, one or both hands on forehead area, signals pressurized problem thinking, whereas had on chin or mouth region correlates to deeper or extended range problems.
The following are related quotes:
- “I divide men into two lots. They are freethinkers, or they are not freethinkers. I am not thinking of the freethinkers who form a political party in Germany, nor of the agnostic English Freethinkers, but am using the word in its simplest meaning. Freethinkers are those who are willing to use their minds without prejudice, and without fearing to understand things that clash with their own customs, privileges, or beliefs. This state of mind is not common, but it is essential for right thinking; where it is absent, discussion becomes worse than useless. A man may be a Catholic, a Frenchman, or a capitalist, and yet be a freethinker; but if he puts his Catholicism, his patriotism, or his interest above his reason, and will not give the latter free play where those subjects are touched, he is not a freethinker. His mind is in bondage.”
- “I, too, was originally supposed to become an engineer. But I found the idea intolerable of having to apply the inventive faculty to matters that make everyday life more elaborate — and all, just for dreary money-making. Thinking for its own sake, as in music! When I have no special problem to occupy my mind, I love to reconstruct proofs of mathematical and physical theorems that have long been known to me. There is no goal in this, merely an opportunity to indulge in the pleasant occupation of thinking.”
- — Albert Einstein (1918), "Letter to Heinrich Zangger"
- “Lorentz is the greatest and most powerful thinker I have ever known. I never met Gibbs, but, perhaps, had I done so, I might have placed him beside Lorentz.”
- — Albert Einstein (c.1925/54), aggregate quote
- “Use your brain. Forget about sports as a profession. Sports make you grunt and smell. See, be a ‘thinker’, not a stinker..”
- — Sylvester Stallone (1976), Rocky (character: Apollo Creed) 
- Commins, Saxe; Linscott, Robert. (1947). The World’s Greatest Thinkers (Amz). Random.
- Thinker – Hmolpedia 2020.