"Source investigation." Historians try to puzzle out the sources of an ancient author's work by looking closely at clues like the author's dates (if known), biases exhibited in all or parts of his work, odd enthusiasms, or opinions which seem more appropriate to an author of an earlier age. (For example, if an E2 writeup were to speak of the dangers of Soviet communism today, you'd quickly guess that the noder had looked at literature from the cold war era.) The game is played by ransacking lists of authors who wrote (or probably wrote) at an earlier date and looking for ones who seem to match the picture called for by the clues gathered from the work of the author under investigation. Sometimes the evidence does not point to someone known but to a putative "author X" who matches it.

Quellenforschung flourished because ancient historians were indifferent about citing their sources, whereas moderns are keen to know which 'facts' an ancient author reports can be trusted and how much. Knowing an ancient historian's source would offer a control on the reliability of his data, and there is nothing wrong in principle with wanting to know who an ancient author's sources were.

An excessive (not to say obsessive) form of Quellenforschung characterized ancient historiography in the latter part of the 19th and earlier part of the 20th centuries and first grew in the fertile soil of the great German universities. By the time its excesses caused it to be downgraded as a method it had been practiced in all parts of the world. It is now in severe disrepute despite some undeniable successes (if you are interested in the "Q" source of the Gospels, you are touching on one of the more successful results of Quellenforschung. That "Q" is shorthand for Quelle, "source"--an example of an "author X"). Among the problems with it are the following:

1) It can blind a researcher to idiosyncratic modes of writing by the ancient author. If an ancient historian has an entertaining, flowing style in general and you encounter a dense patch of hard analytic reasoning, is that copied from a source with a different style, or has your author just suddenly focussed more closely on his topic? Conflicting opinions or deviations from uniformity may wrongly be attributed to the influence of a source author.

2) There was a "Gesetz der Quellenbenützung" (law of source use) advanced by a scholar named Heinrich Nissen in the mid-19th century. He posited that ancient authors tended to use only one source at a time, and this rapidly crystallized into the working assumption that ancient authors basically only could consult one source at a time. The justification was that if you use a papyrus scroll, it will fill up your desk (or workspace) as it unrolls, and that having more than one scroll open at once would lead to difficulty. I've simplified a long discussion almost to the point of caricature here, but even in its least extreme form the idea of such a "law" of source use is problematic because the assumption that an author used only one source at a time is dangerously convenient and so seductive that it offers a specious justification which scholars were all too quick to grab at. This line of thought takes no consideration of those plucky individuals who somehow stumbled through using more than one scroll at a time; authors who didn't think like we do in terms of verbatim quotes but relied on their memory; the perhaps natural human tendency to mix what you already knew in with what you are taking from a source; and the possibility of a mixture of source materials not just from juggling scrolls, but from notes taken in separate reading campaigns and then brought together in producing a final text.

3) I linked to this node from Appian of Alexandria because he is the poster child of an ancient historian who was vivisected by modern scholars seeking his vitals--his heart was a 'reliable' discussion of the Roman civil wars supposedly taken from eyewitness Asinius Pollio; his liver was special material on the triumviral period garnered from Augustus' memoirs; no brain was found. None of these putative sources survives in any form permitting us to see if Appian actually used them. Worst of all, Appian himself was lost in the piecing out of his history, while the parts taken from it were pieced back together into Frankenstein monster reconstructions of Pollio's lost history, etc.

For a worthwhile account of the dissapointments of Quellenforschung (in a discussion on Appian), have a look at the first couple of pages of a good article in English by Brian C. McGing, entitled "Appian's Mithridateios." It is in a series called Aufstieg und Niedergang der römischen Welt (ANRW)--pages 496-522 of volume 34, part 1 of the portion of ANRW devoted to the Roman Empire (II). (The volume cover will say II 34.1--gotta love those German partitioning schemes.)

German readers can go right to the source and read a particularly hair-raising account of a botched vivisection in the regrettable Ernst Kornemann's 1921 article "Die unmittelbare Vorlage von Appians Emphylia," in the journal Klio, volume 17 (1921) 33-43. You'll need access to a pretty good university research library (or interlibrary loan) to get that one. If you can look into the face of the gorgon and live, try Kornemann's 1898 "Die historische Schriftstellerei des C. Asinius Pollio, zugleich ein Beitrag zur Quellenforschung über Appian und Plutarch," in Jahrbücher für classische Philologie Supplementband 22 (1896) 557-691, which set historiographical study of all three ancient authors mentioned in its title back by 50 years.