Huygens, the most celebrated mathematician and physicist of his epoch (1629-95), takes it for granted that all the planets are inhabited (he does not speak of the Sun or stars), and that consequently all of them offer the conditions vital and essential to us, that is to say air and water. He easily refutes the objections based upon the distance of the planets from the Sun. He supposes that the water on the other planets will have quite other qualities than ours; on Mercury, for instance, it would only boil at a very high temperature, and on Saturn--at his time the outermost planet known--it would not freeze at the lowest temperature then conceivable. The mass of Jupiter compared with that of our own globe would suggest that its air must be very dense and that its inhabitants could swim in it; but the Jovians would easily accommodate themselves to this state of things. He supposed that the mind of the inhabitants of the other planets was about the same as ours and that their organism was analogous to our own; for what would be the object of the Sun illuminating those other planets if their inhabitants had no eyes? One might indeed think that different species of reasonable beings might exist, but not on the same planet, for they would be in mutual conflict, would struggle for supremacy, and do each other every kind of harm. He gives a very naïve refutation of an opinion given before him that the height of the inhabitants would have to be inversely as the volume of the planet and consequently the men on Jupiter would not be bigger than the mice on the earth. This, he says, is not possible, because such small beings would not, as astronomers, be capable of using large telescopes. An important question, according to Huygens, is whether the intelligence of the inhabitants has any relation to their distance from the Sun. He inclines to believe that the inhabitants of Mercury are much more intelligent than we on account of the greater force and vitality of their spirit due to the greater heat of the Sun, but this is not confirmed by what happens on our own globe. The same reason would lead one to believe that the inhabitants of Jupiter are much less intelligent than ourselves, although its four satellites would offer to the mind material for profitable astronomical studies. What is also evident is the small value of a purely logical and philosophical reasoning based on insufficient premises. We see how a man of sense and judicious spirit can be led to absurd conclusions when he is burdened with preconceived ideas and incomplete knowledge. We shall have several more occasions to bring out this fact, and that is why we have avoided pretending to resolve the question we have put. The deductions which we formulate to-day may be reversed to-morrow by new discoveries furnished either by experience or by theory.