A further indication of increasing severity may be found in a bull issued by Julius III [Sidenote: 1550] who complained that authors licensed to read heretical {421} books for the purpose of refuting them were more likely to be seduced by them, and who therefore revoked all licenses given up to that time.
[Sidenote: September, 1557] When the Roman Inquisition issued a long list of volumes to be burnt publicly, including works of Erasmus, Machiavelli and Poggio, this might be considered the first Roman Index of Prohibited Books; but the first document to bear that name was issued by Paul IV.

[Sidenote: 1559] It divided writings into three classes: (1) Authors who had erred _ex professo_ and whose whole works were forbidden; (2) Authors who had erred occasionally and some of whose books only were mentioned; (3) Anonymous books.

In addition to these classes 61 printers were named, all works published by whom were banned.

The Index strove to be as complete as possible.

Its chief though not its only source was the catalogue of Louvain.