312 SCIENCE PROGRESS 



they respectively originated differ considerably in national characteristics ; the 

 personal temperaments of the two discoverers are widely asunder ; and yet they 

 possess many features in common. 



A new scientific theory never disposes at once of all difficulties, and serious 

 objections can generally be raised. Addressed, as it generally would be, to 

 scientific men, and expressed in technical language, it invites criticism in detail, 

 and it may be years before some crucial experiment or conclusive argument is 

 found. When confined to the discussion of specialists, its progress is necessarily 

 slow. But great generalisations always contain an element that appeals to a wider 

 public, only that public must be reached. Where there is a conflict, the scale may 

 be turned by the literary power of an exponent, or some striking new fact that 

 stirs the imagination. Galileo was able to supply both. 



Three stages may be recognised in the progress of our ideas on the constitution 

 of the universe. In the first, the earth is considered to be the centre round 

 which everything revolves ; in the second, the sun becomes the centre of the 

 planetary system and of the universe ; in the third, the sun itself is only one 

 member of a vast stellar system. The passage from the second to the third stage 

 was easy. The great struggle came in establishing the second. Yet the idea 

 that the earth revolves round itself and round the sun is an old one. It is said to 

 have been held by Pythagoras, and it was certainly taught by a number of Greek 

 and Indian philosophers, but Aristotle pronounced against it. We may here 

 recognise a fundamental difference in the manner of judging the merits of a theory. 

 We may consider it to be merely an embodiment of a collection of facts, or we 

 may require something more of it. It used to be said that the theory ought to 

 explain the facts ; and though no one would now express it in this form, we never- 

 theless expect a theory to possess some guiding idea which binds the facts 

 together. Otherwise a catalogue of detached statements would do equally well. 

 To the majority of the Greek philosophers this connecting-link was the important 

 matter, and Aristotle believed that he had disposed of the heliocentric doctrine by 

 pointing to the behaviour of heavy bodies on the surface of the earth. They all 

 tend towards its centre, and the earth itself must, therefore, tend towards its own 

 centre. The idea that there may be a similar and more general tendency of 

 all bodies towards each other did not occur to him. 



Copernicus (1473-1573) detached himself entirely from any claim to furnish an 

 explanation. Though this has been looked upon as a merit, it also proved to be 

 his weakness, for he abandoned all possibility of influencing public opinion. He 

 showed that, considered merely as a kinematic problem, it was simpler to place 

 the sun in the centre of the planetary system. But he insisted so much on the 

 purely formal side of his fundamental hypothesis that one is led to believe him 

 to have been influenced more by the fear of stirring up religious opposition than 

 by philosophical insight. The heliocentric doctrine thus remained for more than 

 half a century longer a purely academic subject of discussion. 



Galileo and his contemporary Gilbert were the first scientific investigators in 

 our present meaning of the term, combining philosophical insight with a craving 

 for the extension of knowledge by direct experiment. The history of the mental 

 development of such pioneers is of entrancing interest. Galileo's father was 

 distinguished as a musician, whose writings show him to have been well acquainted 

 with Greek and Roman literature. The son was sent to a classical school, but it 

 was intended that he should earn his livelihood in commerce. The future mapped 

 out for him could not satisfy his ambitions, and when he showed extraordinary 

 ability in mastering the ancient languages, logic, and dialectics, it was decided to 



