The misunderstanding which had begun before Aristotle parted from his royal pupil, but which had not prevented the good offices of the latter, increased towards the end of his career. One of the first occasions seems to have been offered by the philosopher, who, having published his works on physics and metaphysics, received from Alexander, who was piqued at his having divulged to the world the valuable knowledge which he had obtained from him in his youth, the following letter:-- "Alexander to Aristotle, wishing all happiness. You have done amiss in publishing your books on the speculative sciences. In what shall I excel others if what you taught me privately be communicated to all? You know well that I would rather surpass mankind in the more sublime branches of learning than in power. Farewell." This epistle exhibits the king as a very exclusive personage; and, joined to what history has recorded of his actions, tends to show that selfishness, however refined or disguised, was the main source of his insatiable ambition. One of the sincerest pleasures of a great mind is to communicate to others all the blessings that it possesses. On other occasions he appeared to entertain a wish to mortify the philosopher by exalting his rival Xenocrates, who had nothing to recommend him besides a respectable moral character. It has even been asserted by some, that the conqueror, after he had put Callisthenes to death, intended the same fate for Aristotle. This Callisthenes was a kinsman and disciple of the other, through whose influence, it is said, he was appointed to attend the king on his Asiatic expedition. His republican sentiments and independent spirit, however, rendered him an indifferent courtier; while his rude and ill-timed reflections finally converted him into an object of suspicion or dislike. The conspiracy of Hermolaus affording Alexander a plausible pretext for getting rid of his uncourtly monitor, he caused him to be apprehended and put to death. Some say that he was exposed to lions, others that he was tortured and crucified; but, in whatever way he met his end, it is generally agreed that his life was sacrificed to gratify the enmity of his sovereign. Aristotle naturally espoused the cause of his relative, and from that period harboured a deep resentment against his destroyer. It has even been alleged that he was privy to the supposed design of murdering the victorious prince; but of this there is no satisfactory evidence.