This vigour of understanding is displayed in many processes of deductive reasoning, in the power of seizing some general principle underlying diverse phenomena, in the use of analogies by which he illustrates the argument and advances from known to unknown causes and from things within the cognisance of our senses to those beyond their range, and in the clearness and variety of his observation.
His system cannot be called either purely inductive or purely deductive, though it is more of the former than of the latter. He argues with great force both from a large and varied mass of facts to general laws and from general principles to facts involved in them. The best examples of his power of following abstract ideas into their consequences may be found in the first two books, where he establishes the existence of vacuum, the infinity of space and of the atoms, the limitations of the form of the atoms and the like. The reasoning at i. 298-328 where the existence of invisible bodies is established affords a good instance of his power of recognising a common principle involved in a great number and variety of phenomena.
The vigour with which he reasons from known to unknown facts and causes may be judged most fairly by his arguments on the progress of society, where he is more on an equality with modern speculation. He discards, altogether, as might be expected, the fancies concerning a heroic or a golden age, and assumes as his data the facts of human nature as observed in his own day. The grounds from which he starts, his method of reasoning, and the nature of his conclusions remind a reader of the positive tendencies of Thucydides, as they are displayed in the introduction to his history. The importance of personal qualities, such as beauty, strength, and power of mind, in the earliest stage of civil society, the influence of accumulated wealth at a later period, the causes of the establishment and overthrow of tyrannies and of the rise of commonwealths in their room, are all set forth with a degree of strong sense and historical sagacity, such as no other Roman writer has shown in similar investigations. The inferiority even of Tacitus in his occasional digressions into the philosophy of history is very marked. On such topics, where the data were accessible to the natural faculties of observation and inference, and where conclusions were sought which, without aiming at definite certainty, should yet be true in the main, the reader of Lucretius has no sense of that wasted ingenuity which he often feels in following the investigations into some of the primary conditions of the atoms, the component elements of the soul, the process by which the world was formed, or the causes of electric or volcanic phenomena.
Lucretius makes a copious, and often a very happy use, of analogies, both in the illustration of his philosophy, and in passages of the highest poetical power. Some of the most striking of the former kind have already been noticed as sources of error, or at least of disguising ignorance, in his reasoning, viz., those founded on the supposed parallel between the world and the human body; others again are employed with force and ingenuity in support of various positions in his argument. Among these may be mentioned his comparison of the effect of various combinations of the same letters in forming different words, with that of the various combinations of similar atoms in forming different objects in nature. So too the ceaseless motion of the atoms is brought visibly before the imagination by the analogy of the motes dancing in the sunbeam. There is something striking in the comparison of the human body immediately after death to wine 'cum Bacchi flos evanuit,' and again, in that of the relation of body and soul to the relation of frankincense and its odour—
E thuris glaebis evellere odorem
Haud facile est quin intereat natura quoque eius.[404]
But this faculty of his understanding is in general so united with the imaginative feeling through which he discerns the vital indentity of the most diverse manifestations of some common principle, that it can best be illustrated in connexion with the poetical, as distinct from the logical, merits of the work.
So also it is difficult to separate his faculty of clear, exact, and vivid observation from his poetical perception of the life and beauty of Nature. His powers of observation were, however, stimulated and directed by scientific as well as poetic interest in phenomena. From the wide scope of his philosophy he was led to examine the greatest variety of facts, physical as well as moral. His sense of the immensity of the universe led him to contemplate the largest and widest operations of Nature,—such as the movements of the heavenly bodies, the recurrence of the seasons, the forces of great storms, volcanoes, etc.; while, again, the theory of the invisible atoms drew his attention to the minutest processes of Nature, in so far as they can be perceived or inferred without the appliances of modern science. Thus, for instance, in a long passage beginning—
Denique fluctifrago suspensae in litore vestes,[405]
he shows by an accumulation of instances that there are many invisible bodies, the existence of which is inferred from visible effects. In other places he draws attention to the class of facts which have been the basis of the modern science of geology,—such as the mark of rivers slowly wearing away their banks,—of walls on the sea-shore mouldering from the long-continued effects of the exhalations from the sea,—of the fall of great rocks from the mountains under the wear and tear of ages.