 It appears to follow from this analysis that as industry becomes more mechanised, using more constant capital and less variable capital, the rate of profit ought to fall. For as a proportion less capital will be advanced on labour, and only labour can create value. In Capital Volume 3 Marx does indeed make the prediction that the rate of profit will fall over time, and this is one of the factors which leads to the downfall of capitalism. (However, as pointed out by Paul Sweezy in The Theory of Capitalist Development (1942), the analysis is problematic.) A further consequence of this analysis is a difficulty for the theory that Marx did recognise, and tried, albeit unsuccessfully, to meet also in the manuscripts that make up Capital Volume 3. It follows from the analysis so far that labour-intensive industries ought to have a higher rate of profit than those which use less labour. Not only is this empirically false, it is theoretically unacceptable. Accordingly, Marx argued that in real economic life prices vary in a systematic way from values. Providing the mathematics to explain this is known as the transformation problem, and Marx’s own attempt suffers from technical difficulties. Although there are sophisticated known techniques for solving this problem now there is a question about the degree to which they do rescue Marx’s project. If it is thought that the labour theory of value was initially motivated as an intuitively plausible theory of price then when the connection between price and value is rendered as indirect as it is in the final theory, the intuitive motivation of the theory drains away. Others consider this to be a superficial reading of Marx, and that his general approach allows us to see through the appearances of capitalism to understand its underlying basis, which need not coincide with appearances. How Marx’s theory of capitalism should be read remains an active area of scholarly debate (Heinrich 2012).