2. In those cases in which we have a division of the science which teaches us the doctrine of the causes, as well as one which states the rules which the effects follow, I have, in the _History_, distinguished the two portions of the science by certain terms. I have thus spoken of _Formal_ Astronomy and _Physical_ Astronomy. The latter phrase has long been commonly employed to describe that department of Astronomy which deals with {120} those forces by which the heavenly bodies are guided in their motions; the former adjective appears well suited to describe a collection of rules depending on those ideas of space, time, position, number, which are, as we have already said, the _forms_ of our apprehension of phenomena. The laws of phenomena may be considered as _formulæ_, expressing results in terms of those ideas. In like manner, I have spoken of Formal Optics and Physical Optics; the latter division including all speculations concerning the machinery by which the effects are produced. Formal Acoustics and Physical Acoustics may be distinguished in like manner, although these two portions of science have been a good deal mixed together by most of those who have treated of them. Formal Thermotics, the knowledge of the laws of the phenomena of heat, ought in like manner to lead to Physical Thermotics, or the Theory of Heat with reference to the cause by which its effects are produced;--a branch of science which as yet can hardly be said to exist. 3. What _kinds of cause_ are we to admit in science? This is an important, and by no means an easy question. In order to answer it, we must consider in what manner our progress in the knowledge of causes has hitherto been made. By far the most conspicuous instance of success in such researches, is the discovery of the causes of the motions of the heavenly bodies. In this case, after the formal laws of the motions,--their conditions as to space and time,--had become known, men were enabled to go a step further; to reduce them to the familiar and general cause of motion--mechanical force; and to determine the laws which this force follows. That this was a step in addition to the knowledge previously possessed, and that it was a real and peculiar truth, will not be contested. And a step in any other subject which should be analogous to this in astronomy;--a discovery of causes and forces as certain and clear as the discovery of universal gravitation;--would undoubtedly be a vast advance upon a body of science consisting only of the laws of phenomena. {121}