Apart from sacrifices, prophecy and divination played a great part in the life and religion of the Greeks. A distinction made by the ancients themselves was between prophecy by art and without art. Prophecy without art was regarded as inspiration of a human being by the divine spirit, and was not dependent on external signs or on the interpretation of an experienced person. There were three kinds: ecstasy, in which the gift of prophecy was communicated to a human being without his own assistance by divine strength and power; dreams, in which the gods revealed directly to men their will or coming events; and thirdly, the oracles, which were of a somewhat different character, being connected with professional prophecy. They were also regarded as direct revelations of the will of the god, so the mode in which this was expressed differed a good deal according to the various oracles; but the questioner was not immediately inspired, as in ecstasy and dreams, but required a mediator, one who was alone able to interpret the revelations of the gods. Of these three classes, the least important during the historic period is ecstasy; the seers in the real sense of the word, whom we so often meet with in legends, had no importance later on. The second kind, the dream oracle, is of far greater importance. The idea that dreams were communications from the gods, no less than other oracles and signs, was so universally adopted that it not only took firm root in the popular belief, but was shared by educated men, even by those who had more or less discarded the old belief in the gods. The ancient writers give us numerous accounts of portentous dreams; unlucky dreams were averted by religious ceremonies, sacrifices to the gods who could turn away ill fortune, sprinkling with holy water, etc. It was usual to pray for prophetic dreams, and, as we have already seen, these were specially produced by sleeping in the temple of Aesculapius, though they often required interpretation afterwards at the hands of the priests. It is a very old belief that dreams reveal the will of the gods, not directly and immediately, but in the form of parables or images, which require special comprehension and secret knowledge, and thus the interpretation of dreams became an especial art, which led to a whole literature of dream-books (remains of which have been preserved to us; in particular the dream-book of Artemidorus, dating from the second century A.D.), and to the profession of interpreters, who, although not held in especial honour, were yet greatly sought after by all classes of the community.