THREE:The modern doctrine of evolution, while relying largely on the fertility of multiplied chances, is not obliged to assume such an enormous number of simultaneous coincidences as Epicurus. The ascription of certain definite attractions and repulsions to the ultimate particles of matter would alone restrict their possible modes of aggregation within comparatively narrow limits. Then, again, the world seems to have been built up by successive stages, at each of which some new force or combination of forces came into play, a firm basis having been already secured for whatever variations they were capable of producing. Thus the solar system is a state of equilibrium resulting from the action of two very simple forces, gravitation and heat. On the surface of the earth, cohesion and chemical affinity have been superadded. When a fresh equilibrium had resulted from their joint energy, the more complex conditions of life found free scope for their exercise. The transformations of living species were similarly effected by variation on variation. And, finally, in one species, the satisfaction of its animal wants set free those more refined impulses by which, after many experiments, civilisation has been built up. Obviously the total sum of adaptations necessary to constitute our actual world will have the probabilities of its occurrence enormously increased if we suppose the more general conditions to be established prior to, and in complete independence of, the less general, instead of limiting ourselves, like the ancient atomists, to one vast simultaneous shuffle of all the material and dynamical elements involved.It is, at any rate, certain that the successors of Aenesidmus adhered to the standpoint of Pyrrho. One of them, Agrippa, both simplified and strengthened the arguments of the school by reducing the ten Tropes to five. The earlier objections to human certainty were summed up under two heads: the irreconcilable conflict of opinions on all subjects; and the essential relativity of consciousness, in which the percipient and the perceived are so intimately united that what things in themselves are cannot possibly be discovered. The other three Tropes relate to the baselessness of reasoning. They were evidently suggested by Aristotles remarks on the subject. The process of proof cannot be carried backwards ad infinitum, nor can it legitimately revolve in a circle. Thus much had already been admitted, or rather insisted on by the great founder of logic. But the Sceptics could not agree to Aristotles contention, that demonstration may be based on first principles of self-evident certainty. They here fell back on their main argument; that the absence of general agreement on every point is fatal to the existence of such pretended axioms. A still further simplification was effected by the reduction of the five Tropes to twothat all reasoning rests on intuition, and that mens intuitions are irreconcilably at variance with one another.300 As against true science, the sceptical Tropes are powerless, for the validity of its principles has nothing to do189 with their general acceptance. They are laid before the learner for his instruction, and if he chooses to regard them as either false or doubtful, the misfortune will be his and not theirs. But as against all attempts to constrain belief by an appeal to authority, the Tropes still remain invincible. Whether the testimony invoked be that of ancient traditions or of a supposed inward witness, there is always the same fatal objection that other traditions and other inward witnesses tell quite a different story. The task of deciding between them must, after all, be handed over to an impersonal reason. In other words, each individual must judge for himself and at his own risk, just as he does in questions of physical science.
