Capital, Volume Three: The Process of Capitalist Production as a Whole by Karl Marx

The distinction between currency and capital, as Tooke, Wilson, and others draw it, whereby the differences between medium of circulation as money, as money-capital generally, and as interest- bearing capital (moneyed capital in the English sense) are thrown together pell-mell, comes down to two things. [viii]

Currency circulates on the one hand as coin (money), so far as it promotes the expenditure of revenue, hence the traffic between the individual consumers and the retail merchants, to which category belong all merchants who sell to the consumers – to the individual consumers as distinct from productive consumers or producers. Here money circulates in the function of coin, although it continually replaces capital. A certain portion of money in a particular country is continually devoted to this function, although this portion consists of perpetually changing individual coins. In so far as money promotes the transfer of capital, however, either as a means of purchase (medium of circulation) or as a means of payment, it is capital. It is, therefore, neither its function as a means of purchase, nor that as a means of payment, which distinguishes it from coin, for it may also act as a means of purchase between one dealer and another so far as they buy from one another in hard cash, and also as a means of payment between dealer and consumer so far as credit is given and the revenue consumed before it is paid. The difference is, therefore, that in the second case this money not only replaces the capital for one side, the seller, but is expended, advanced, by the other side, the buyer, as capital. The difference, then, is in fact that between the money-form of revenue and the money-form of capital, but not that between currency and capital, for a certain quantity of money circulates in the transactions between dealers as well as in the transactions between consumers and dealers. It is, therefore, equally currency in both functions. Tooke's conception introduces confusion into this question in various ways: