"Each period of human achievement has its phases of spring, culmination, and decline; and it is in its decline that the leafless tree comes to judgment. In the unloveliness of decay, the Middle Ages are as other ages have been; as our own will be; but in those ages there was more than one outburst of life; more than once the enthusiasm of the youth of the West went out to explore the ways of the realm of ideas; and if we believe ourselves at last to have found the only thoroughfare, we owe this knowledge to those who before us traveled the uncharted seas. If we have inherited a great commerce and dominion of science, it is because their argosies had been on the ocean and their camels on the desert. _Discipulus est prioris posterior dies;_ man cannot know all at once; knowledge must be built up by laborious generations. In all times, as in our own, the advance of knowledge is very largely by elimination and negation; we ascertain what is not true, and we weed it out. To perceive and respect the limits of the knowable, we must have sought to transgress them. We can build our bridge over the chasm of ignorance with stored material in which the thirteenth century was poor indeed; we can fix our bearings where then was no foundation; yet man may be well engaged when he knows not the ends of his work; and the schoolmen in digging for treasure cultivated the field of knowledge, even for Galileo and Harvey, for Newton and Darwin. Their many errors came not of indolence, for they were passionate workers; not of hatred of light, for they were eager for the light; not of fickleness, for they wrought with unparalleled devotion; nor indeed of ignorance of particular things, {307} for they knew many things. They erred because they did not know, and they could not know the conditions of the problems which, as they emerged from the cauldron of war and from the wreck of letters and science, they were nevertheless bound to attack, if civil societies worthy of the name were to be constructed."