[212] _Oeuv._, ii. 12, 13, Sec. 6. See the same idea in the Encyclopaedia, above, vol. i. pp. 225-227. We cannot wonder that when Comte drew up his list of the hundred and fifty volumes that should form the good Positivist's library in the nineteenth century, he should have placed Diderot's _Interpretation of Nature_ on one side of Descartes' _Discourse on Method_, with Bacon's _Novum Organum_ on the other. The same spirit finds even stronger and more distinct expression in a later aphorism:--"Since the reason cannot understand everything, imagination foresee everything, sense observe everything, nor memory retain everything; since great men are born at such remote intervals, and the progress of science is so interrupted by revolution, that whole ages of study are passed in recovering the knowledge of the centuries that are gone,--to observe everything in nature without distinction is to fail in duty to the human race. Men who are beyond the common run in their talents ought to respect themselves and posterity in the employment of their time. What would posterity think of us if we had nothing to transmit to it save a complete insectology, an immense history of microscopic animals? No--to the great geniuses great objects, little objects to the little geniuses" (Sec. 54). Diderot, while thus warning inquirers against danger on one side, was alive to the advantages of stubborn and unlimited experiment on the other. "When you have formed in your mind," he says, "one of those systems which require to be verified by experience, you ought neither to cling to it obstinately nor abandon it lightly. People sometimes think their conjectures false, when they have not taken the proper measures to find them true. Obstinacy, even, has fewer drawbacks than the opposite excess. By multiplying experiments, if you do not find what you want, it may happen that you will come on something better. _Never is time employed in interrogating nature entirely lost_" (Sec. 42). The reader will not fail to observe that this maxim is limited by the condition of verifiableness. Of any system that could not be verified by experience Diderot would have disdained to speak in connection with the interpretation of nature. This, of course, did not prevent him from hypothesis and prophecy which he himself had not the means of justifying. For example, he said that just as in mathematics, by examining all properties of a curve we find that they are one and the same property presented under different faces, so in nature when experimental physics are more advanced, people will recognise that all the phenomena, whether of weight, or elasticity, or magnetism, or electricity, are only different sides of the same affection (Sec. 44). But he was content to leave it to posterity, and to build no fabric on unproved propositions. In the same scientific spirit he penetrated the hollowness of every system dealing with Final Causes: