The sum of the first two books, in which the principles of the atomic philosophy are methodically unfolded and illustrated, is, accordingly, to this effect:--that all things have their origin in, and are sustained by, the various combinations and motions of solid elemental atoms, infinite in number, various in form, but not infinite in the variety of their forms,--not perceptible to our senses, and themselves devoid of sense, of colour, and of all the secondary properties of matter. These atoms, by virtue of their ultimate conditions, are capable only of certain combinations with one another. These combinations have been brought about by perpetual motion, through infinite space and through all eternity. As the order of things now existing has come into being, so it must one day perish. Only the atoms will permanently remain, moving unceasingly through space, and forming new combinations with one another. These first principles being established, the way is made clear for the true explanation, according to natural laws, of those phenomena which give rise to and maintain the terrors of superstition. The third book treats of the nature of the mind, and of the vital principle. As it is by the fear of death, and of eternal torment after death, that human life is most disturbed, it is necessary to explain the nature of the soul, and to show that it perishes in death along with the body. The mind and the vital principle are parts of the man as much as the hands, feet, or any other members. The mind is the directing principle, seated in the centre of the breast. The vital principle is diffused over the whole body, obedient to and in close sympathy with the mind. The power which the mind has in moving the body proves its own corporeal nature, as motion cannot take place without touch, nor touch without the presence of a bodily substance. The soul (including both the mind and vital principle) is, therefore, material, formed of the finest or minutest atoms, as is proved by the extreme rapidity of its movement, and by the fact that there is nothing lost in appearance or weight immediately after death:-- Quod simul atque hominem leti secura quies est Indepta atque animi natura animaeque recessit, Nil ibi libatum de toto corpore cernas Ad speciem, nil ad pondus: mors omnia praestat Vitalem praeter sensum calidumque vaporem[10]. Four distinct elements enter into the composition of the soul--heat, wind, calm air, and a finer essence 'quasi anima animai.' The variety of disposition in men and animals depends on the proportion in which these elements are mixed.