This then is the Method of Science. It consists in forming a law or statement by only looking at a small portion of the facts; then, when the other facts come in, the law or statement gradually fades away again. Conrad Gessner and other early zoologists began by classifying animals according to the number of their horns! Political Economy begins by classifying social action under a law of Supply and Demand. When people believed that the earth was flat, they generalised the facts connected with the fall of heavy bodies into a conception of "up and down." These were two opposite directions in space. Heavy bodies took the "downward"; it was their nature. But in time, and as fresh facts came in, it became impossible to group animals any longer by their horns; "up and down" ceased to have a meaning when it was known that the earth was round. Then fresh laws and statements had to be formed. In the last-mentioned case--it being conceived that the earth was the centre of the universe--the new law supposed was that all heavy bodies tended to the centre of the earth as such. This was all right and satisfactory for a while; but presently it appeared that the earth was _not_ the centre of the universe, and that some heavy bodies--such as the satellites of Jupiter--did not in fact tend to the centre of the earth at all. Another lump of ignorance (which had enabled the old generalisation to exist) was removed, and a new generalisation, that of universal gravitation, was after a time formed. But it is probable that this law is only conceived of as true through our ignorance; nay it is certain that belief in its truth presents the gravest difficulties.