Now, if we adopt the scientific theory of Marx that labour pure and simple is the sole producer of wealth, and that labour is productive in proportion to the hours devoted to it, how has it happened--this is our crucial question--that the amount of labour which produced seven at one period should produce thirty-three at another? How are we to explain the presence of the additional twenty-six? The answer of Marx, and of those who reason like him, is that, owing to the development of knowledge, mechanical and chemical especially, and the consequent development of industrial methods and machinery, labour as a whole has itself become more productive. But to say this is merely begging the question. To what is this development of knowledge, of methods, and of machinery due? Is it due to such labour as that of the "untirable human animals," to which Mill refers as an example of labour in its intensest form? In a word, does ordinary labour, or the industrial effort of the majority, contain in itself any principle of advance at all? We must, in order to do justice to any theory, consider not only the points on which its exponents lay the greatest stress, but also those which they recognise as implied in it, or which we may see to be implied in it ourselves. And if we consider the theory of Marx in this way, we shall see that labour, in the sense in which he understands the word, does contain principles of advance which are of two distinguishable kinds. One of these is recognised by Marx himself. Just as, when he says that labour is the sole productive agency, he assumes the gifts of nature, which provide it with something to work upon, so, when he conceives of labour as the effort of hand and muscle, he assumes a human mind behind these by which hand and muscle are directed. Such being the case, he expressly admits also that mind is in some cases a more efficient director than in others, and is able to train the hands and muscles of the labourer, so that these acquire the quality which is commonly called skill. Ruskin, who asserted, like Marx, that labour is the sole producer, used in this respect a precisely similar argument. He defined skill as faculty which exceptional powers of mind impart to the hands of those by whom such powers are possessed, from the bricklayer who, in virtue of mere alertness and patience, can lay in an hour more bricks than his fellows, up to a Raphael, whose hands can paint a Madonna, while another man's could hardly be trusted to distemper a wall evenly.