"Perhaps you're correct," Carstairs admitted, after a pause.
"Well, I want to join the happy band. Shove your knowledge forward, having due regard to the manner of your doing so, that it does not defeat its own ends. And that is wisdom; you're paid for the combined product of your knowledge and your wisdom. Wisdom is the most scarce, the most valuable, and the most difficult to acquire: it is the knowledge of the use of knowledge. Do you see?"
"They bear the relation to each other of theory and practice in engineering."
"Not quite. Theory is an effort of the imagination, either a spontaneous effort of your own, based on known facts, or an assimilation of the results of other men's practice as recorded by them in books. The sources of error are twofold; the limits of your own imagination, your own conception of the other man's description, and the limit of the other man's gift of expression and explanation. Practice is your own conception and remembrance of what you yourself have personally experienced. Both are knowledge; wisdom is distinct from either."
Carstairs smiled. "Well, it's your wisdom I doubt, not your knowledge. I mean to say, that my application of my knowledge to my conception of your application of your knowledge, as expressed by you in the present discussion, leads me to doubt the accuracy of your application of your knowledge to the case under discussion. The possible sources of error being in my imagination or your expression, and as my imagination is a fixed quantity, unless you can improve your expression, I shall fail to coincide with you. How's that?"
"That's very good." Darwen took a deep breath and laughed. "Let me have another shot. Who gets the most money, the successful professor or the successful business man?"
"The successful business man."
"Hear, hear! That's because he's selling wisdom, while the professor is selling knowledge."
"I disagree, on two points. Number one, the business man sells necessities, boots for instance; the professor sells luxuries, quaternions, for instance." Carstairs paused and quoted, "'What I like about quaternions, sir, is that they can't be put to any base utilitarian purpose.'"
"Quaternions, my dear chap——"