Thinking (n.) is a process of inquiry, of looking into things, of investigating.
Thinking (n.) is an active, persistent, and careful consideration of any belief or supposed form of knowledge in the light of the grounds that support it and the further consequences to which it tends.
Thinking (n.) is done only when confronted with a problem.
Thinking (n.) is placing some portion of the world in jeopardy.
Thinking (n.) is restricted to matters not directly perceived: To what we do not see, smell, hear, or touch.
Thinking (n.) is an act requiring the presence of a note of invention, as distinct from faithful record of observation.
Thinking (n.) is an activity whose origin is some perplexity, confusion or doubt.
Thinking (n.) is a necessarily personal act. To 'think for oneself' is tautological in that, unless one does it for one's self, it isn't thinking.
Thinking (n.) is that operation in which present facts suggest other facts (or truths) in such a way as to induce belief in the latter upon the ground or warrant of the former.