According to Marx, capitalists take advantage of the difference between the labour market and the market for whatever commodity the capitalist can produce. Marx observed that in practically every successful industry, input unit-costs are lower than output unit-prices. Marx called the difference "surplus value" and argued that it was based on surplus labour, the difference between what it costs to keep workers alive and what they can produce. Although Marx describes capitalists as vampires sucking worker's blood, he notes that drawing profit is "by no means an injustice" and that capitalists cannot go against the system. The problem is the "cancerous cell" of capital, understood not as property or equipment, but the relations between workers and owners – the economic system in general.