48 THE HISTORY OF BIOLOGY 



the soul. Following Aristotle, with whose theory of the finality of all 

 things he nevertheless sharply disagrees, he assumes three kinds of soul, 

 animus, mens, and anima'^ — that is, the spirit, the understanding, and the 

 soul or life-principle. He does not, however, consistently differentiate be- 

 tween these three categories, but discusses them mostly as one single idea. 

 In quality the soul is material, an organ, like the rest, formed of extremely 

 small atoms distributed throughout the body, very mobile, and therefore 

 easily dispersed. These atoms are of three determinable kinds: warmth, air, 

 and "aura," a more rarified kind of air corresponding to the "pneuma" of 

 Hippocrates. But besides the three categories of atoms just named, the soul 

 contains still a fourth component, which has no name, but which forms the 

 real percipient, the consciousness in the soul, and whose atoms are the small- 

 est and most mobile. They give impulse to the other soul-atoms and thereby 

 indirectly to the movements of the body. These component parts of the soul, 

 being variously commingled, produce the varying soul-characteristics in 

 different individuals and make themselves felt in different degrees in the same 

 individual in different states of mind; heat in anger, cold air in terror, etc. 

 The soul, which in life is contained by the body, as a vessel contains whatever 

 is kept therein, dissolves at death into the simplest component parts and is 

 annihilated. The immortality of the soul as maintained by Plato and his 

 disciples is attacked by Lucretius with passionate intensity. Again and again 

 he seeks to prove that this, combined with a belief in gods, is the cause of all 

 human miseries. Sense-perceptions are, according to Lucretius, due to things' 

 giving off from their surface a kind of light particles which, formed like the 

 things themselves, float about in space and influence the organs of sense. As 

 a proof that such images are given off he cites, inter alia, the change of skin 

 in snakes and insects. All sensations — sight, hearing, smell, and taste — 

 are thus excited by different atoms which affect the organs of the body. The 

 ideas arising herefrom are caused by a mass of still more subtle images of 

 things, floating about in space even after the things themselves have dis- 

 appeared. Thus one sees in imagination images of individuals long since 

 dead, and owing to the images' sometimes coalescing, one receives impres- 

 sions of creatures which have never existed in reality; for instance, through 

 the coalescing of a horse and a human image is produced the mythical 

 centaur. Through such images still remaining in the soul dreams arise. Primi- 

 tive as these sensory physiological speculations are, they are nevertheless 

 accompanied by a number of extremely striking observations regarding differ- 

 ent kinds of sensations. In particular Lucretius discussed in detail sensations 

 in the sphere of sexual life, which he describes with a curious mixture of 



^ These names, which Lucretius undoubtedly himself invented on the Greek model, are 

 again found in the psychological terminology of the Middle Ages, and even in Swedenborg. 



