[18] Since the above was written there has certainly been a great change, and the dogmatic confidence in the verity of the scientific "laws" has now (1920) almost disappeared. [19] Such fictions, however, are (I need not say) quite necessary as our only means of thinking out, however imperfectly, the problems before us (1920). [20] It is not generally realised how feeble a force gravitation is. It is calculated (Encycl. Brit., Art. Gravitation) that two masses, each weighing 415,000 tons, and placed a mile apart, would exert on each other an attractive force of only one pound. If one, therefore, was as far from the other as the moon is from the earth, their attraction would only amount to 1/57,600,000,000th of a pound. This is a small force to govern the movement of a body weighing 415,000 tons! and it is easy to see that a slight variation in the law of the force might for a long period pass undetected, though in the course of hundreds of centuries it might become of the greatest importance. [21] As another instance of the same thing, let me quote a passage from Maxwell's _Theory of Heat_, p. 31; the italics are mine: "In our description of the physical properties of bodies as related to heat we have begun with solid bodies, as those which we can _most easily handle_, and have gone on to liquids, which we can keep in open vessels, and have now come to gases, which will escape from open vessels, and which are generally _invisible_. This is the order which is most natural in our first study of these different states. But as soon as we have been made familiar with the most prominent features of these different conditions of matter the most _scientific_ course of study is in the _reverse_ order, beginning with gases, on account of the greater simplicity of their laws, then advancing to liquids, the more complex laws of which are much more imperfectly known, and concluding with the little that has been hitherto discovered about the constitution of solid bodies." That is to say that Science finds it easier to work among gases--which are invisible and which we can know little about--than among solids, which we are familiar with and which we can easily handle! This seems a strange conclusion, but it will be found to represent a common procedure of Science--the truth probably being that the laws of gases are not one whit _simpler_ than the laws of liquids and solids, but that on account of our knowing so much less about gases it is easier for us to _feign_ laws in their case than in the case of solids, and less easy for our errors to be detected.