Money is a fungible proxy for man's ability to reason, labor, create and produce.
I'm not sure I would call it a definition. I would call it a statement, but would modify it somewhat:
"Ideally, money should be a fungible proxy for one man's ability to reason, labor, create and produce that allows him to exchange for the ability to reason, labor, create and produce of others."
I think these statements just elaborate a little on the "medium of exchange" and "measure of value" definitions of money. Just a complex way of saying that money should allow for the honest exchange of fish, bread, and beer between the fisherman, baker, and brewer.