Space can be filled with organised matter or matter capable of life distributed almost uniformly, without being specially destined for any particular body, since we must assume a commencement for each. The surface of the heavenly bodies receives the organisable matter which, when its finds the necessary conditions, develops and forms living beings. It is clear that in this case the presence of organised beings on all the heavenly bodies capable of entertaining life is not merely a probability but a certainty. Scheiner considers the three hypotheses as equally acceptable. The first, he says, is a matter of sentiment. It does not solve the question, since it would involve a Divine Will. Out of the three hypotheses the last two would solve our problem in an absolutely affirmative manner, whereas the first leaves it undecided. “We only wish to prove one thing by our argument, and that is that the opinion that habitable heavenly bodies are really inhabited is much more probable than the contrary opinion, and this authorises us to continue to develop our thesis.” THE CONDITIONS OF LIFE The author here puts a fundamental question. What is life and what is living matter! Libraries have been written on this subject but all to no purpose, for we are as unable to seize the essence of life and of living matter as that of gravitation, for instance, though the latter appears to us infinitely simpler in its manifestations than what has been called vital force. We only know that on our Earth vital force is united to a special form of matter called organised matter, and that when this organised matter disappears it ceases to exist as vital force and transforms itself--since energy is indestructible--into other forms of energy. It is not of material importance whether vital force is regarded as a special force or a special aspect of a known force, _e.g._ electricity. It follows from this intimate association of vital force with living matter that the vital manifestations (nutrition, growth, reproduction, etc.) can only take place under conditions in which living matter can exist; in all other cases the manifestations of life cease and death ensues, or the vital force becomes latent until favourable conditions return. Our problem of the habitability of heavenly bodies is therefore limited to the question of celestial bodies on which conditions are such that living matter can exist in a permanent form. We must therefore enquire first of all what those conditions are, after which we shall be able to use our astronomical resources to find whether these conditions are represented on the other centres of condensation of matter in the universe. The conditions necessary to life are the more numerous the more complicated the structure of organic matter. The maximum of requirements is therefore attained in the higher animals and in man.