The Druids had immunity from military service and from the payment of tribute.

These privileges drew many into training for the profession, some of their own accord, others at the instance of parents and relatives.

While in training they were said to learn by heart a large number of verses, and some went so far as to spend twenty years in their course of preparation.
The Druids held it wrong to put their religious teaching in writing, though, in almost everything else, whether public or private affairs, they made use of Greek letters.

Caesar thought that they discouraged writing on the one hand, lest their teaching should become public property; on the other, lest reliance upon writing should lessen the cultivation of the memory.

To this risk Caesar could testify from his own knowledge.