The process by which children, and persons born blind who have been operated upon, learn to see, the single vision of the double sensation of two eyes, the double vision and double touch which occur when the organs of sense have been displaced from their usual position, the upright appearance of objects while the picture on the retina is upside down, the attributing of colour to the outward objects, whereas it is merely an inner function, a division through polarisation, of the activity of the eye, and lastly the stereoscope,-all these are sure and incontrovertible evidence that perception is not merely of the senses, but intellectual-that is, pure knowledge through the understanding of the cause from the effect, and that, consequently, it presupposes the law of causality, in a knowledge of which all perception-that is to say all experience, by virtue of its primary and only possibility, depends.
Haurrengan eta ebakuntza egin zaien jaiotzatiko itsuengan ikusten ikastea, bi begiekin bikoitz sentituaren ikuste sinplea, ikuste bikoitza eta ukitze bikoitza, sentsu-lanabesa beren ohiko posizioetatik deslekutzean, objektuen agerpen zuzena, horien imajina begian alderantzikaturik dagoela, koloreen transposizioa, zeina begiaren ekintzaren barne-funtzio bat baita, banaketa polar bat, kanpoko objektuen gain eta, azkenik, estereoskopia,-horiek guztiak froga finko eta ezeztaezinak dira, begiespen oro ez soilik sentsual, baizik eta intelektual delakoarena, h. d. efektutiko kausaren adimen-ezagutza hutsarena, ondorioz kausalitatearen legea suposatzen duena, zeinaren ezagutzaren mendean baitago begiespen oro, eta horrekin bere ahaltasun lehen eta osoaren esperientzia oro, ez alderantziz kausalitatearen legearen ezagutza esperientziatik, hori Humeren eszeptizismoa litzateke, zeina honela bakarrik ezeztatzen baita.