The Stoics were a group of philosophers who placed great emphasis upon ethics in comparison with logic and physics. They looked on the world from the pessimistic side and made themselves happy by becoming martyrs. They taught that suffering, the endurance of pain without complaint, was the highest virtue. To them logic was the science of thought and of expression, physics was the science of nature, and ethics the science of the good. All ideas originated from sensation, and perception was the only criterion of truth. "We know only what we perceive (by sense); only those ideas contain certain knowledge for us which are ideas of real objects." The soul of man was corporeal and material, hence physics and metaphysics were almost identical. There is much incoherency in their philosophy; it abounds in paradoxes. For instance, it recognizes sense as the criterion and source of knowledge, and asserts that reason is universal and knowable. Yet it asserts that there is no rational element in sense that is universal. It confuses individual human nature and universal nature, though its final result was to unite both in one concept. The result of their entire philosophy was to create confusion, although they had much influence on the practical life. The Sceptics doubted all knowledge obtained by the senses. There was no criterion of truth in the intellect, consequently no knowledge. If truth existed it was in conduct, and thus the judgment must be suspended. They held that there was nothing that could be determined of specific nature, nothing that could be of certainty. Eventually the whole Greek philosophy went out in scepticism. The three schools, the sceptic, the Epicurean, and the stoic, though widely differing in many ways, agreed upon one thing, in basing their philosophy on subjectivity, on mind rather than on objective nature.