However, independent source material from this first phase of exegetical activity, that may verify or falsify that view, is virtually nonexistent. All we know about the early period is from later ascriptions. Later works, from the middle of the second century AH at the earliest, claim to contain the exegesis of earlier authorities. But whether or not these claims are valid cannot be checked, because no objective criteria can be applied. Or to put it differently, original material, such as papyri from this early period that could substantiate these claims, has as yet not been found. Leemhuis, "Origins and Early Development of the tajszr Tradition," p. 14.