One of the primary objectives of scholars of early Welsh over the past century and a half has been to identify which poems within the voluminous corpus of poetry preserved in later medieval Welsh manuscripts were originally composed and written down during the pre-Norman period. This was one of the chief interests of Sir Ifor Williams, and others have since taken up the baton. Philology, broadly defined, is the essential basis for any argument that the composition and writing of a poem should be dated to a time many centuries before the time of its preservation, though historical arguments of varying quality have also been employed. The philological arguments have hitherto been based mainly on matters of phonology, since far more is known about the phonology of early Welsh than about its morphology or syntax. But the philological arguments for the dating of poems suspected to derive from the pre-Norman period have proved contentious, partly because of how comparatively slight the verifiable linguistic differences between Old Welsh and Middle Welsh are. A recent survey of the debate surrounding the poems attributed to Aneirin and the ‘historical’ Taliesin has concluded that no phonological arguments have yet been identified that would serve to prove whether the poems were composed as early as the sixth century, even though it remains plausible, from a phonological standpoint, that many of them could have been.