302 THE PRINCIPLES OF SCIENCE. 



not form any idea of such various creatures, but their 

 existence is not unlikely?. 



We now know that many metals and other elements 

 never found in organic structures are yet capable of form- 

 ing compounds, with substances of vegetable or animal 

 origin. It is therefore just possible that at different tem- 

 peratures creatures formed of different but analogous com- 

 pounds might exist, but it would seem indispensable that 

 carbon should still form the basis of organic structures ; 

 for we have no analogies to lead us to suppose that in 

 the absence of that complex element, life can exist. Could 

 we find globes surrounded by atmospheres resembling our 

 own in temperature and composition, we should be almost 

 forced to believe them inhabited, but the probability of 

 any analogical argument decreases rapidly as the condi- 

 tion of a globe diverges from that of our own. The Cardi- 

 nal Nicholas de Cusa held long ago that the moon was 

 inhabited, but the absence of any appreciable atmosphere 

 renders the existence of inhabitants highly improbable. 

 Speculations resting upon weak analogies hardly belong 

 to the scope of true science, and can only be tolerated as 

 an antidote to the far worse dogmatism which would 

 assert that the thousand million of persons on earth, or 

 rather a small fraction of them, are the sole objects of 

 care of the Power which designed this limitless Universe. 



Failures of Analogy. 



So constant is the aid which we derive from the use of 

 analogy in all attempts at discovery or explanation, that 

 it is most important to observe in what cases it may lead 

 us into difficulties. That which we expect by analogy to 

 exist may 



P 'System of the World,' vol. ii. p. 326. 'Essai Philosophique,' p. 87. 



