The existence of physical evil gave the Stoics little trouble, since they refused to regard it as an evil at all, as will be seen in treating of their ethical system. It was enough for them to refer evils of this kind—diseases, for instance—to natural causes, and to regard them as the inevitable consequences of causes framed by nature to serve a definite purpose. [416] Still, they did not fail to point out that many things only become evil by a perverted use, [417] and that other things, ordinarily regarded as evils, are of the greatest value. [418] [(b) Existence of moral evil] Greater difficulty was found by the Stoics to beset the attempt to justify the existence of moral evil, and the difficulty was enhanced in their case by the prevalence and intensity of moral evil in the world [419] according to their view. By their theory of necessity they were prevented from shifting the responsibility for moral evil from natural law or deity on to man, which is one way out of the difficulty. In not altogether eschewing this course, and yet refusing to allow to deity any participation in evil, and referring evil to the free will and intention of man, [420] they acted as other systems of necessity have done before, [421] reserving the final word. The real solution which they gave to the difficulty is to be found partly in the assertion that even the deity is not able to keep human nature free from faults, [422] and partly in the consideration that the existence of evil is necessary, as a counterpart and supplement to good, [423] and that, in the long run, evil will be turned by the deity into good. [424] [(c) Connection between virtue and happiness.]