In his statement of the doctrine of the _Clinamen_, or slight declension in the motion of the atoms, so as 'to break the chain of fate,' he attributes to them a power analogous to volition in living beings. This doctrine is suggested by the necessity of explaining contingency in Nature and freedom in the movements of sentient beings. We are, as in all attempts to account for creation, forced back on the thought of an ultimate unexplained power in virtue of which things have been created and are maintained in being. The Lucretian hypothesis of the atoms, even if it were accepted as the most reasonable explanation of the original constitution of matter, is, by itself, altogether inadequate as a key to the secret of Nature. It cannot be shown either how these atoms succeeded in arranging themselves in order, or how from their negative properties all positive life has been produced. The explanation of physical phenomena given in the four last books, as to the nature of our bodies and souls,--as to the action of outward things on the senses,--the origin and existence of the sun and moon, the earth and the living beings upon it, etc., although professedly deduced from the principles established in the first two books, are really reached independently. They are either immediate inferences from the obvious intimations of sense, or they are the suggestions of analogy. The weakness as well as the strength of ancient science lay in its perception of analogies. The mind of Lucretius was both under the influence of earlier analogical conceptions, and also shows great boldness and originality in the logical and poetical apprehension of 'those same footsteps of Nature, treading on diverse subjects or matters.' But, in common with the earlier enquirers of Greece, he trusts too implicitly to their guidance through all his daring adventure. He seems to believe that the hidden properties of things are as open to discovery through this 'lux sublustris' of the imagination, as through the 'lucida tela' of the reason. To take one prominent instance of this influence, it is remarkable how, in his explanation of our mundane system, he is both consciously and unconsciously guided by the analogy of the human body. Even Lucretius, living in the very meridian of ancient science, cannot in imagination absolutely emancipate himself from the associations of mythology. He is indeed conscious of the inconsistency of attributing life and sense to the earth: yet not only does he speak poetically of Earth being the creative mother, Aether the fructifying father of all things, but his whole conception of the creation of the world is derived from a supposed likeness between the properties of our terrestrial and celestial systems, and those of living beings. Thus we read-- Undique quandoquidem per caulas aetheris omnis Et quasi per magni circum spiracula mundi Exitus introitusque elementis redditus extat.[397] Of the growth of plants and herbage it is said--