On Philosophers' Use of 'Cash'
Philosophers use the term “cash” in a special way, as when they say, “This [concept] needs to be cashed out.” It’s another way of saying “analyzed.” I don’t know this, but I suspect the term derives from cash, as in money. To cash a check is to reduce it to (transform it into) money. To cash out a concept is to reduce it to (transform it into) other, more familiar, concepts.
My conjecture is that the current use of 'cash' and 'cash out' by philosophers is in large part due to Wilfrid Sellars' use of these expressions, although I can't point to any specific passages in Sellars' writings. It is probable that Sellars was adapting William James' use of 'cash-value.' Describing the pragmatic method in "What Pragmatism Means," James writes, "You must bring out of each word its practical cash-value, set it at work within the stream of your experience." (William James, Pragmatism and The Meaning of Truth, ed. Ayer, Harvard University Press, 1981, pp. 31-32.)
A pragmatist is one who, being told that such-an-such an idea is true, wants to know "the truth's cash-value in experiential terms." (p. 169) According to James, "True ideas are those we can assimilate, validate, corroborate and verify. False ideas are those that we cannot." (Ibid.)
If so, then to cash a concept is not merely to analyze it, but to analyze it into experiential terms. Thus Hume cashed out the concept of causation into such experiential terms as spatiotemporal contiguity, temporal succession, and constant conjunction. The sub-concept of necessary connection remained a 'check' that could not be cashed in experiential terms, and so was cashiered -- if you will allow me a pun. If we were simply analyzing the ordinary concept of causation, we would have to mention the sub-concept of necessary connection, that the cause necessitates the effect, bringing it into existence. By cashing out the concept of causation in experiential terms, however, what Hume was doing was replacing the ordinary concept of causation with one that satisfies empiricist strictures.
So although I wouldn't disagree that 'cash out' is used in the general sense of 'analyze,' I would suggest that it carries the more specific flavor of 'analyze into experiential terms.'