Money as the problem

Will & Ariel Durant (Ancient Greece)
"By the time of Plato's death (347 B.C.) his hostile analysis of Athenian democracy was approaching apparent confirmation by history. Athens recovered wealth, but this was now commercial rather than landed wealth; industrialists, merchants, and bankers were at the top of the reshuffled heap. The change produced a feverish struggle for money, a pleonexia, as the Greeks called it--an appetite for more and more...." - Will & Ariel Durant, The Lessons of History, p.74.
H.G. Wells (Ancient Rome, after the fall of Carthage)
"In this curiously interesting century of Roman history we find man after man asking, 'What has happened to Rome?' Various answers are made--a decline in religion, a decline from the virtues of the Roman forefathers, Greek 'intellectual poison,' and the like. We, who can look at the problem with a large perspective, can see that what had happened to Rome was 'money.' - H.G. Wells, The Outline of History, page 366.
Sir Edwin Sandys (1621)
"The best instance to my knowledge of a typically mercantilist discussion of a state of affairs of this kind is the debates in the English House of Commons concerning the scarcity of money, which occurred in 1621, when a serious depression had set in.... Sir Edwin Sandys ... stated ... that looms were standing idle for want of money...." Heckscher, quoted by Keynes in his General Theory, Chapter 23, p.347.
Sir William Petty (May 27, 1623 – December 16, 1687)
"Petty believed that there was a certain amount of money that a nation needed to drive its trade. Hence it was possible to have too little money circulating in an economy ...[or]... too much...." - Wikipedia.
Adam Smith (1776)
"When paper is substituted in [place] of gold and silver money, the quantity of materials, tools, and maintenance, which the whole circulating capital can supply [is] increased.... An operation of this kind has... been performed in Scotland.... The business of the country is almost entirely carried on by means of the paper of those different banking companies.... But though the conduct of all those companies has not been unexceptional, and has accordingly required an act of parliament to regulate it, the country, notwithstanding, has evidently derived great benefit from their trade. I have heard it asserted, that the trade of the city of Glasgow doubled in about fifteen years... and that the trade of Scotland has more than quadrupled...." - Adam Smith, The Wealth of Nations, Book 2, Chapter 2, pp.393-94.
J. S. Crawford (1818)
"...issuing bank notes is actually a public robbery, and is the cause of most of the evils that torment society and humanity..." - Seventy-six, J. S. Crawford, in Cause Of, and Cure For, Hard Times, p.52. Available as a Google book.
John Stuart Mill (1848)
"There cannot, in short, be intrinsically a more insignificant thing, in the economy of society, than money ... and ... it only exerts a distinct and independent influence of its own when it gets out of order." - John Stuart Mill, quoted by Milton & Rose Friedman in Free to Choose, Chapter 9, p.249.
Silvio Gesell (1929)
"The criterion of good money, of an efficient instrument of exchange, is: 1. That it shall secure the exchange of goods - which we shall judge by the absence of trade depressions, crises and unemployment." - Silvio Gesell, The Natural Economic Order. Quoted in Wikipedia.
Arthur Shipman (1977)
"Excess demand is the cause of inflation. When there's too much money, you have excess demand for goods. You have 'demand-pull' inflation. When there's not enough money, you have excess demand for money. You get 'cost-push' inflation.... You get inflation no matter what's wrong with the money supply. That's the whole problem." - A.F. Shipman, Small Change, page 19.
Milton Friedman (1992)
"Money is so crucial an element in the economy, yet also largely an invisible one, that even what appear to be insignificant changes in the monetary structure can have far-reaching and unanticipated effects." - Milton Friedman, Money Mischief, Chapter 11, pp.261-262.
Jordan MacLeod (2009)
"Money is at the heart of it all" - Jordan Bruce MacLeod, New Currency: How Money Changes the World As We Know It. From the Note to the Reader.