It might be said, generally, that the argument of Lucretius was an attempt to give a philosophical description of Nature before the advent of physical science. But, as a means of throwing light on the inadequacy of such speculations, it may be well to consider in detail some of those points where the argument most obviously fails in premises, method, and results.
The ancient as well as the modern enquirer into the truth of things was confronted with the question of the origin of all our knowledge. Is knowledge obtained originally through the exercise of the reason or the senses, or through their combined and inseparable action? To this question Lucretius distinctly answers, that the senses are the foundation of all our knowledge.[18] They are our 'prima fides': the basis not only of all sound inference, but of all human conduct. The very conception of the meaning of true and false is derived from the senses:—
Invenies primis ab sensibus esse creatam
Notitiam veri neque sensus posse refelli[19].
But besides the direct action of outward things on the senses, he admits the power of certain images to make themselves immediately present to the mind (iv. 722-822), and also a certain immediate apprehension or intuition of the mind (iniectus animi) into things beyond the cognisance of sense[20]. Thus there is no actual inconsistency with his principles in claiming the power of understanding the properties and configuration of the atoms, which are represented as lying below the reach of our senses—
Omnis enim longe nostris ab sensibus infra
Primorum natura iacet.
But of the mode of operation of this 'intuition of the mind' there is no criterion. The doctrine of the properties, shapes, motions, etc. of the atoms is a creation of the imagination, suggested by certain analogies from sensible things, but incapable of being verified by the senses, which he regards as the only sure foundations of knowledge.
But even on the supposition that the existence and properties of the atoms had been satisfactorily established, no adequate explanation is offered of their relation to the facts of existence. The same difficulty is encountered at the outset of this as of all other ancient systems of ontology, viz. how to pass from the eternal and immutable forms of the atoms to the variety and transitory nature of sensible objects. This is the very difficulty which Lucretius himself urges against the system of Heraclitus,—
Nam cur tam variae res possint esse requiro,