The art of engraving coins did not flourish alone in Alexandria; painters and sculptors flocked to Egypt to enjoy the favours of Ptolemy. Apelles, indeed, whose paintings were thought by those who had seen them to surpass any that had been before painted, or were likely to be painted, had quarrelled with Ptolemy, who had known him well when he was the friend and painter of Alexander. Once when he was at Alexandria, somebody wickedly told him that he was invited to dine at the royal table, and when Ptolemy asked who it was that had sent his unwelcome guest, Apelles drew the face of the mischief-maker on the wall, and he was known to all the court by the likeness. It was, perhaps, at one of these dinners, at which Ptolemy enjoyed the society of the men of letters, or perhaps when visiting the philosophers in their schools, that he asked Euclid if he could not show him a shorter and easier way to the higher truths of mathematics than that by which he led the pupils in the Museum; and Euclid, as if to remind him of the royal roads of Persia, which ran by the side of the highroads, but were kept clear and free for the king's own use, made him the well-known answer, that there was no royal road to geometry. Ptolemy lived in easy familiarity with the learned men of Alexandria; and at another of these literary dinners, when Diodorus, the rhetorician, who was thought to have been the inventor of the Dilemma, was puzzled by a question put to him by Stilpo, the king in joke said that his name should be Cronus, a god who had been laughed at in the comedies. Indeed, he was so teased by Ptolemy for not being able to answer it, that he got up and left the room. He afterwards wrote a book upon the subject; but the ridicule was said to have embittered the rest of his life. This was the person against whom Callimachus, some years later, wrote a bitter epigram, beginning "Cronus is a wise man." Diodorus was of the sceptical school of philosophy, which, though not far removed from the Cyrenaic school, was never popular in Alexandria. Among other paradoxes he used to deny the existence of motion. He argued that the motion was not in the place where the body moved from, nor in the place that the body moved to, and that accordingly it did not exist at all. Once he met with a violent fall which put his shoulder out of joint, and he applied to Herophilus, the surgeon, to set it. Herophilus began by asking him where the fall took place, whether in the place where the shoulder was, or in the place where it fell to; but the smarting philosopher begged him to begin by setting his limb, and they would talk about the existence of motion after the operation.