He remembers, i.e. he knows (for remembrance is but the reviving of some past knowledge) that he was once certain of the truth of this proposition, that the three angles of a triangle are equal to two right ones.
Gogoratzen du, hau da, ezagutzen du (oroitzapena iraganeko ezagutza bat berritzea baino ez da) "triangelu baten hiru angeluek bi angelu zuzen osatzen dute" dioen proposizioaren egiaz behiala ziur zegoela.