But when I am dealing with a period or an event for which evidence is lacking or deficient, then obviously it is a sound criterion of criticism to accept the probable and not to presuppose the improbable. Common sense and general experience are nowhere more necessary than in their application, whether in a court of law or in the study of history, to those problems whose difficulty consists in the absence of direct proof. [Footnote: For instance, there is no contemporary account mentioning London during the last half of the fifth and nearly all the sixth century. Green, Freeman, Stubbs, say (making it up as they go along) that London ceased to exist: disappeared! Then (they assert) after a long period of complete abandonment it was laboriously cleared by a totally new race of men and as laboriously rebuilt on exactly the same site. The thing is not physically impossible, but it is so exceedingly improbable that common sense laughs at it.] Remembering all this, let us first set down what is positively known from record with regard to the fate of Britain in the hundred and fifty years of “the gap.” We begin by noting that there were many groups of German soldiery in Britain before the Pirate raids and that the southwest was—whether on account of earlier pirate raids or on account of Saxon settlers the descendants of Roman soldiers—called “the Saxon shore” long before the Imperial system broke down. Next we turn to documents. There is exactly one contemporary document professing to tell us anything at all of what happened within this considerable period, exactly one document set down by a witness; and that document is almost valueless for our purpose. It bears the title, _De Excidio Brittaniæ Liber Querulus_. St. Gildas, a monk, was its author. The exact date of its compilation is a matter of dispute—necessarily so, for the whole of that time is quite dark. But it is certainly not earlier than 545. So it was written one hundred years after the beginning of that darkness which covers British history for one hundred and fifty years; most of the Roman regulars had been called away for a continental campaign in 410. They had often so left the island before. But this time the troops sent out on expedition did not return. Britain was visited in 429 and 447 by men who left records. It was not till 597 that St. Augustine landed. St. Augustine landed only fifty years at the most after Gildas wrote his _Liber Querulus_, whereas the snapping of the links between the Continent and southeastern Britain had taken place at least a hundred years before.