We have just seen that, apart from money-capital, circulating capital is only another name for commodity-capital. But to the extent that labour power circulates in the market, it is not capital, no form of commodity-capital. It is not capital at all; the labourer is not a capitalist, although he brings a commodity to market, namely his own skin.

Vol. II, Ch. X, p. 211. - Das Kapital (Buch II) (1893)