Despite Marx's stress on critique of capitalism and discussion of the new communist society that should replace it, his explicit critique is guarded, as he saw it as an improved society compared to the past ones (slavery and feudalism). Marx never clearly discusses issues of morality and justice, but scholars agree that his work contained implicit discussion of those concepts. Marx's view of capitalism was two-sided. On one hand, in the 19th century's deepest critique of the dehumanising aspects of this system he noted that defining features of capitalism include alienation, exploitation and recurring, cyclical depressions leading to mass unemployment. On the other hand, he characterised capitalism as "revolutionising, industrialising and universalising qualities of development, growth and progressivity" (by which Marx meant industrialisation, urbanisation, technological progress, increased productivity and growth, rationality and scientific revolution) that are responsible for progress. Marx considered the capitalist class to be one of the most revolutionary in history because it constantly improved the means of production, more so than any other class in history and was responsible for the overthrow of feudalism. Capitalism can stimulate considerable growth because the capitalist has an incentive to reinvest profits in new technologies and capital equipment. 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. At the same time, Marx stressed that capitalism was unstable and prone to periodic crises. He suggested that over time capitalists would invest more and more in new technologies and less and less in labour. Since Marx believed that profit derived from surplus value appropriated from labour, he concluded that the rate of profit would fall as the economy grows. Marx believed that increasingly severe crises would punctuate this cycle of growth and collapse. Moreover, he believed that in the long-term, this process would enrich and empower the capitalist class and impoverish the proletariat. In section one of "The Communist Manifesto", Marx describes feudalism, capitalism and the role internal social contradictions play in the historical process:  Marx believed that those structural contradictions within capitalism necessitate its end, giving way to socialism, or a post-capitalistic, communist society: Thanks to various processes overseen by capitalism, such as urbanisation, the working class, the proletariat, should grow in numbers and develop class consciousness, in time realising that they can and must change the system.