Abandoning our efforts to understand the universe as a whole, let us glance for a moment at the relation of life to the universe we know. The old view that every point of light in the sky represented a possible home for life is quite foreign to modern astronomy. The stars themselves have surface-temperatures of anything from 1650 degrees to 30,000 degrees or more, and are of course at far higher temperatures inside. By far the greater part of the matter of the universe is at a temperature of millions of degrees, so that its molecules are broken up into atoms, and the atoms are broken up, partially at least, into their constituent parts. Now the very concept of life implies duration in time; there can be no life where atoms change their make up millions of times a second and no pair of atoms can ever stay joined together. It also implies a certain mobility in space, and these two implications restrict life to the small range of physical conditions in which the liquid state is possible. Our survey of the universe has shewn how small this range is in comparison with that exhibited by the universe as a whole. It is not to be found in the stars, nor in the nebulae out of which the stars are born. We know of no type of astronomical body in which the conditions can be favourable to life except planets like our own revolving round a sun. Now, to the best of our present knowledge, planets are very rare. We have seen how a single star cannot of itself produce planets. A family of planets must have two parents; it only comes into being as the result of the close approach of two stars, and stars are so sparsely scattered in space that it is an inconceivably rare event for one to pass near to a neighbour. On the Tidal Theory, explained on p. 236, planets cannot be born except when two stars pass within about three diameters of one another. As we know how the stars are scattered in space, we can estimate fairly closely how often two stars will approach within this distance of one another. The calculation shews that even after a star has lived its life of millions of millions of years, the chance is still about a hundred thousand to one against its being a sun surrounded by planets.