The beginning of modern philosophy is signaled by Descartes' (1596–1650) fundamental questioning of all knowledge. The instrument of examination was the methodological doubt which unmasked all traditional knowledge as ultimately unfounded and left no certainty except itself: the process of doubting. Such reflection was not novel in itself and had been undertaken in much the same way by Augustine some 1300 years before. The significance of Descartes' doubt lay in the new concept of knowledge which was the positive aspect and the end of this doubt. For something to be admitted as knowledge, it had to justify itself not in being impressive, useful, edifying, or hoary, but in furnishing incontrovertible credentials as to its origin. Incontrovertible, that meant for Descartes: universally valid for a precisely defined domain of entities; in short, only scientific knowledge would from now on count as knowledge proper.