I had not decided beforehand on the number of my chapters; it seems that there will be thirteen. If these pages have readers to whom the number thirteen is distressing, I beg of them at once to dismiss this feeling by saying: “He who objects to sit down thirteen at table, acknowledges by this that he does not believe in a supreme intelligence, superior to his own, which governs the world.” * * * * * Science, religion, reason, and faith, these four words form the circle in which all intellects move, now more than ever; on this all the world is agreed, but all the world does not know what the greatest thinkers have understood by these four words. If we do not wish to deserve the title given to that collective being, “the man in the street,” the best means of avoiding it is to acknowledge openly that there are many unexplained problems facing us, and that man exists in order to do his part in solving them. Humanity is not composed of individuals who have been poured forth from a horn of plenty, its destiny cannot therefore be to diffuse itself over the surface of the earth without the means of knowing why it is there. An ancient Greek said once that the gods were ready to sell all kinds of good things to mortals but at a high price, at the cost of hard work. If then we can only acquire the promised good things by the aid of hard work, our thoughts carry us at once to science, and we ask what can this science do, upon which we so pride ourselves in this century, to explain the motive of our existence? PHYSICS In proportion as physical science studies this universe, so it recognises more and more clearly that its most general phenomenon is vibration, a periodical movement, which propagates itself in waves succeeding each other at regular intervals. We have all noticed the effect produced by drops of rain falling on water which the absence of wind leaves perfectly tranquil. Each drop forms a circle, but the causes of perturbation of an aqueous surface are infinite; the dip of an insect, the leap of a fish, all ceaselessly cause new circles, which follow each other, become wider, and finally lose themselves in each other under our eyes; the water is apparently a prey to shivering fits; this is a type of the vibrations whose percussions are felt by the whole world. We are all, body and soul, subject to the law of vibrations, each sense recognises its power by means of sensations whose various kinds are apprehended by physical science, by the calculation of the number of vibrations which, in a given time, affect differently each of our organs of sensation. Science records the number of vibrations which denote to our skin the exact degree of the external temperature, she counts the millions of vibrations which enable our eyes to see definite colours in the space of a second, and the thousands of vibrations which enable our ears to hear, in the same space of time, well defined sounds.