One thing at any rate is clear, that the Druids and their doctrines, or supposed doctrines, had made a deep impression on the writers of the ancient world.

There is a reference to them in a fragment of Aristotle (which may not, however, be genuine) that is of interest as assigning them a place in express terms both among the Celts and the Galatae.

The prominent feature of their teaching which had attracted the attention of other writers, such as the historian Diodorus Siculus and the Christian theologian Clement of Alexandria, was the resemblance of their doctrine concerning the immortality and transmigration of the soul to the views of Pythagoras.

Ancient writers, however, did not always remember that a religious or philosophical doctrine must not be treated as a thing apart, but must be interpreted in its whole context in relation to its development in history and in the social life of the community in which it has flourished.

To some of the ancients the superficial resemblance between the Druidic doctrine of the soul's future and the teaching attributed to Pythagoras was the essential point, and this was enough to give the Druids a reputation for philosophy, so that a writer like Clement of Alexandria goes so far as to regard the Druids of the 'Galatae' along with the prophets of the Egyptians, the 'Chaldaeans' of the Assyrians, the 'philosophers of the Celts,' and the Magi of the Persians as the pioneers of philosophy among the barbarians before it spread to the Greeks.