3 ON THE QUESTION: ARE THE PLANETS INHABITED?

There are two distinct species of evidence which we are accustomed to employ in the investigation of Natural Truth: the evidence of demonstration and the evidence of analogy.
1
To one or the other of these we have recourse in questions to which that direct evidence afforded by the senses, or by testimony, is inapplicable. The first species of evidence, that of demonstration, is of chief utility in the higher physical and mathematical sciences, its office being to reduce observed phenomena to the dominion of fixed laws, and again from fixed and ascertained laws to deduce the succession of phenomena. This induction of laws and derivation of phenomena has constituted in all the more important sciences the labour of many minds, and of successive ages. Thus in the science of Astronomy, it is to Newton that we owe the discovery of the law of Gravitation. It is to him in part, and to the Astronomers of France in the next age in a greater degree, that we must assign the honour of having traced that law to its remoter consequences. It will be seen that the entire process, whether of induction or derivation, rests on the general principle, that there is an appointed order of nature, that her sequences of cause and effect are invariable. Hence, were our means of analysis sufficiently powerful and comprehensive, it would be possible from a knowledge of the existing state of the material universe, to determine its condition at any future period. One of the questions which I have proposed to consider this evening—the question of the stability and permanency of the existing planetary system, is of this character, and to a certain extent admits of being answered by that species of evidence which I have endeavoured to describe. I do not however, when I come to speak of the question, design to offer any special account of the evidence on which it rests. Such an attempt would certainly fail of its object. I shall on that subject restrict myself to a simple exposition of results.
2