Two problems are involved in the analysis of gnosticism. First comes the criticism of the sources. Since most of our information about gnosis still comes from the church fathers, we must try to determine how reliably such writers as Irenaeus, Hippolytus, and Epiphanius have reproduced their sources. In this connection Sagnard's La gnose valentinienne et le témoignage de saint Irénée (Paris, 1947), is especially useful since it establishes Irenaeus' essential trustworthiness. Second comes the interpretation of the gnostic text once it has been established. At this point there is much disagreement among modern scholars. In general the older way of looking at gnosticism, set forth by such scholars as De Faye, Leisegang, Casey, Nock, and most recently Sagnard, is based on description and historical investigation, with emphasis laid on the search for sources and interrelations. Ideally, proof of the existence of these sources and interrelations is offered, and the proof is based on historical probabilities.