in it, as Aristotle wished to do, the ultimate causes of the motions of the universe, could only mislead. The Aristotelian maxim, which sounds so plausible, and has been so generally accepted, that "to know truly is to know the causes of things," is a bad guide in scientific research. Instead of it we might substitute this: that "though we may aspire to know at last _why_ things are, we must be content for a long time with knowing _how_ they are." 10. Hence if we are asked whether Plato or Aristotle had the truer views of the nature and property of Science, we must give the preference to Plato; for though his notion of a real Intelligible World, of which the Visible world was a fleeting and changeable shadow, was extravagant, yet it led him to seek to determine the forms of the Intelligible Things, which are really the laws of visible phenomena; while Aristotle was led to pass lightly over such laws, because they did not at once reveal the causes which produced the phenomena. 11. Aristotle, throughout his works, takes numerous occasions to argue against Plato's doctrine of Ideas. Yet these Ideas, so far as they were the Intelligible Forms of Visible Things, were really fit objects of philosophical research; and the search after them had a powerful influence in promoting the progress of Science. And we may see in the effect of this search the answer to many of Aristotle's strongest arguments. For instance, Aristotle says that Plato, by way of explaining things, adds to them as many Ideas, and that this is just as if a man having to reckon a large number, were to begin by adding to it another large number. It is plain that to this we may reply, that the adopting the Ideas of Cycles, along with the motions of the Planets, does really explain the motions; and that the Cycles are not simply added to the phenomena, but include and supersede the phenomena: a finite number of Cycles include and represent an infinite number of separate phenomena. To Aristotle's argument that Ideas cannot be the Causes or Principles of Things, we should reply, that though they cannot be this, they may nevertheless be, and must be, the Conditions and Principles of our Knowledge, which is what we want them to be. I have given an account of the main features of Aristotle's philosophy, so far as it concerns the Physical Sciences, in the History of the Inductive Sciences, Book I. FOOTNOTES: [Footnote 20: _Analyt. Prior._ ii. 25.] [Footnote 21: See on this subject Appendix, Essay D.] [Footnote 22: See the chapter on Certain Characteristics of Scientific Induction in the _Phil. Ind. Sc._ or in the _Nov. Org. Renov._] [Footnote 23: _Phil. Ind. Sc._ b. viii. c. i. art. 11, or _Hist. Sc. Id._ b. viii.] [Footnote 24: B. i. c. xi. sect. 2.] [Footnote 25: B. iii. c. i. sect. 9.] [Footnote 26: _De Cælo_, ii. 13.] [Footnote 27: _Ibid._ ii. 10.] [Footnote 28: xii. 8.] [Footnote 29: B. xvi. c. vi.]