What lay beyond the frontier of the known was a speculation inseparable from the spirit of exploration. Children, and people who do not travel, are generally content, when their thoughts stray beyond the paths trodden by their feet, to believe that the greater world is but a continuation on every side of their own environment; indeed, without the help of sight or suggestion, it is almost impossible to believe anything else. If you stand on an eminence in a great plain and think of the unseen country that lies beyond the horizon, trying to visualise it and imagine that you see it, the eye of imagination can only see the continuance or projection of what is seen by the bodily sight. If you think, you can occupy the invisible space with a landscape made up from your own memory and knowledge: you may think of mountain chains and rivers, although there are none visible to your sight, or you may imagine vast seas and islands, oceans and continents. This, however, is thought, not pure imagination; and even so, with every advantage of thought and knowledge, you will not be able to imagine beyond your horizon a space of sea so wide that the farther shore is invisible, and yet imagine the farther shore also. You will see America across the Atlantic and Japan across the Pacific; but you cannot see, in one single effort of the imagination, an Atlantic of empty blue water stretching to an empty horizon, another beyond that equally vast and empty, another beyond that, and so on until you have spanned the thousand horizons that lie between England and America. The mind, that is to say, works in steps and spans corresponding to the spans of physical sight; it cannot clear itself enough from the body, or rise high enough beyond experience, to comprehend spaces so much vaster than anything ever seen by the eye of man. So also with the stretching of the horizon which bounded human knowledge of the earth. It moved step by step; if one of Prince Henry’s captains, creeping down the west coast of Africa, discovered a cape a hundred miles south of the known world, the most he could probably do was to imagine that there might lie, still another hundred miles farther south, another cape; to sail for it in faith and hope, to find it, and to imagine another possibility yet another hundred miles away. So far as experience went back, faith could look forward. It is thus with the common run of mankind; yesterday’s march is the measure of to-morrow’s; as much as they have done once, they may do again; they fear it will be not much more; they hope it may be not much less.