As Arcesilaus renounced all pretensions to the certain determination of any question, he was chiefly employed in examining and refuting the sentiments of others. His principal opponent was his contemporary, Zeno, the founder of the stoical philosophy, which ultimately became the chief of those systems which flourished at Rome. The main point in dispute between Zeno and Arcesilaus, was the evidence of the senses. Arcesilaus denied that truth could be ascertained by their assistance, because there is no criterion by which to distinguish false and delusive objects from such as are real. Zeno, on the other hand, maintained that the evidence of the senses is certain and clear, provided they be perfect in themselves, and without obstacle to prevent their effect. Thus, though on different principles, the founder of the Stoics agreed with the Peripatetics and old Academicians, that there existed certain means of ascertaining truth, and consequently that there was evident and certain knowledge. Arcesilaus, though he did not deny that truth existed, would neither give assent nor entertain opinions, because appearances could never warrant his pronouncing on any object or proposition whatever. Nor did the Stoics entertain opinions; but they refrained from this, because they thought that everything might be perceived with certainty. Arcesilaus, while differing widely from the teachers of the old Platonic Academy in his ideas as to the certainty of knowledge, retained their system concerning the supreme good, which, like them, he placed in virtue, accompanied by external advantages. This was another subject of contest with Zeno, who, as is well known, placed the supreme good in virtue alone,—health, riches, and reputation, not being by him accounted essential, nor disease, poverty, and ignominy, injurious to happiness. The systems promulgated in the old and new Academy, and the stoical Portico, were those which became most prevalent in Rome. But the Epicurean opinions were also fashionable there. The philosophy of Epicurus has been already mentioned while speaking of Lucretius. Moschus of Phœnicia, who lived before the Trojan war, is said to have been the inventor of the Atomic system, which was afterwards adopted and improved by Leucippus and Democritus, whose works, as Cicero expresses it, were the source from which flowed the streams that watered the gardens of Epicurus(362). To the evidence of the senses this teacher attributed such weight, that he considered them as an infallible rule of truth. The supreme good he placed in pleasure, and the chief evil in pain. His scholars maintained, that by pleasure, or rather happiness, he meant a life of wisdom and temperance; but a want of clearness and explicitness in the definition of what constituted pleasure, has given room to his opponents for alleging that he placed consummate felicity in sensual gratification.