Again, let us take an example from Physics--Boyle's law of the compressibility of gases. This law states that, the temperature remaining constant, the volume of a given quantity of gas is inversely proportional to its pressure. It is a law which has been made a good deal of, and at one time was thought to be true, _i.e._, it was thought to be a statement of fact. A more extended and careful observation, however, shows that it is only true under so many limitations, that, like the ellipse in Astronomy, it must be regarded as a convenient fiction and nothing more. It appears that air follows the supposed law pretty well, but not by any means exactly except within very narrow limits of pressure; other gases, such as carbonic acid and hydrogen, deviate from it very considerably--some more than others, and some in one direction and some in the opposite. It was found, among other things, that the nearer a gas was to its liquefying point, the greater was the deviation from the supposed law, and the conclusion was jumped at that the law was true for _perfect_ gases only. This idea of a perfect gas of course involved the assumption that gases, as they get farther and farther removed from their liquifying point, reach at last a fixed and stable condition, when no further change in their qualities takes place--at any rate for a very long time--and Boyle's law was supposed to apply to this condition. Since then, however, it has been discovered that there is an ultra-gaseous state of matter, and on all sides it is becoming abundantly clear that the change in the condition of matter from the liquid state to the ultra-gaseous state is perfectly continuous--through all modifications of liquidity and condensation and every degree of perfection and imperfection of gasiness to the utmost rarity of the fourth state. At what point, then, does Boyle's law really apply? Obviously it applies _exactly_ at only one point in this long ascending scale--at one metaphysical point--and at every other point it is incorrect. But no gas in Nature remains or can be maintained just at one point in the scale of its innumerable changes. Consequently, all we can say is that out of the innumerable different states that gases are capable of, and the innumerable different laws of compressibility which they therefore follow, we could theoretically find one state to which would correspond the law of compressibility called Boyle's law; and that, _if_ we could preserve a gas in that state (which we can't), Boyle's law really _would_ be true just for that case. In other words, the law is metaphysical. It has no real existence. It is a convenient view or fiction, arising in the first place out of ignorance, and only tenable as long as further observation is limited or wilfully ignored.