ratiocinate

järkeillä To use the powers of the mind logically and methodically; to reason.
1887, w:Robert Louis Stevenson|Robert Louis Stevenson, The Treasure of Franchard, ch. 6:

"Observe the relative position," returned the Doctor with a smile. "It is your attitude to believe through thick and thin in one man's judgment—your own. I follow the same opinion, but critically and with open eyes. Which is the more irrational—I leave it to yourself."

"Oh, my dear fellow!" cried Casimir, " . . . Don't rationate with me."