the text or letter by name, or a recognizable form of that text. Examples include a list of accepted biblical books, such as the Muratorian fragment, or the contents of an early manuscript, such as Papyrus 46. Unfortunately, these witnesses are often either damaged or too late in date to provide much help. Implicit references are quotation from Paul, especially indirect or unattributed, or expressing ideas and phrases that appear in his works. This use or reference implies the material quoted was in existence at the time the external evidence was created. For example, the Second Epistle to the Thessalonians is named by Irenaeus in the mid-2nd century, as well as Justin Martyr and Ignatius of Antioch; it is considered unlikely for the surviving version of this letter to have been written after this time. On the other hand, lack of witness by ancient sources suggests a later date, an argument from silence. However, use of this line of reasoning is dangerous, because of the incompleteness of the historical record: many ancient texts are lost, damaged, have been revised or possibly contrived. Historical setting An independently written narrative of Paul's life and ministry, found in the Acts of the Apostles, is used to determine the date, and possible authorship, of Pauline letters by locating their origin within the context of his life. For example, Paul mentions that he is a prisoner in his Epistle to Philemon 1:7; based on this statement, J. A. T. Robinson argued that this captivity was Paul's imprisonment in Caesarea, while W. M. Ramsay identified this as Paul's captivity in Rome, while others have placed the captivity in Ephesus. One difficulty with this position is the limited data available on Paul's historical setting, and this is especially true with the conclusion of the narrative of Acts prior to Paul's death. It also assumes that the book of Acts was written by an actual traveling companion of Paul's. Recent scholarship challenges the validity of using the Acts of the Apostles to determine anything about Pauline letters. Borg and Crossan cautiously comment that "the author of Acts was not concerned with factual inerrancy"; Dewey et al. note in similar vein that "in much of Acts as a whole, Luke gives us historical fiction rather than an historical report." They therefore "recommend against using Acts, either as a whole or in part, as a historical source for Paul's career".