Moreover, Gibbon carefully studied and compared all the primary sources, and it may be urged that he has given a truer, fuller, and more attractive account of the period than can be found in any one of them. His Decline and Fall of the Roman Empire is certainly a work of the highest rank; but, nevertheless, it is only report of others' reports. It is therefore not a primary but a secondary source.
A secondary source is one based on other sources.