At length Diderot concludes by a series of questions which he thinks that philosophers may perhaps count worthy of discussion. What is the difference, for example, between living matter and dead? Does the energy of a living molecule vary by itself, or according to the quantity, the quality, the forms of the dead or living matter with which it is united? We need not continue the enumeration, because Diderot himself suddenly brings them to an end with a truly admirable expression of his sense of how unworthy they are of the attention of serious men, who are able to measure the difference between a wise and beneficent use of intelligence, and a foolish and wasteful misuse of it. "When I turn my eyes," he says, "to the works of men, and see the cities that are built on every side, all the elements yoked to our service, languages fixed, nations civilised, harbours constructed, lands and skies measured--then the world seems to me very old. When I find man uncertain as to the first principles of medicine and agriculture, as to the properties of the commonest substances, as to knowledge of the maladies that afflict him, as to the pruning of trees, as to the best form for the plough, then it seems as if the earth had only been inhabited yesterday. And if men were wise, they would at last give themselves up to such inquiries as bear on their wellbeing, and would not take the trouble to answer my futile questions for a thousand years at the very soonest; or perhaps, even, considering the very scanty extent that they occupy in space and time, they would never deign to answer them at all." II.