The progressive character of the modern nations has been due, then, to the actions and reactions between the spirit of inquiry and the improvement of forms of social organisation; each step in the one respect has reacted upon the other, stimulating further change in the same direction. And the medium through which they have chiefly thus worked upon one another has been the increase of intercourse between men and nations. The spirit of inquiry has urged men on to explore their fellow men and to study foreign nations, and it has provided the means for so doing; the greater mutual knowledge and sympathy thus brought into being have in turn brought greater liberty to the spirit of inquiry, freeing it from the rigid bonds of custom and conservative tradition and enabling it to render human intercourse yet more free and abundant. In this way we reconcile and synthesize the rival theories of the causes of progress, the view that sees in the spirit of inquiry the sole agent of progress and that which attributes it wholly to the improvement of morals and of social organisation. The great commandment, “Thou shalt love thy neighbour as thyself,” pointed the way of all progress; but great and beautiful as it was, it could not immediately avail to break the bonds of the human spirit, the bonds of ignorance and fear; only gradually through increase of knowledge could man learn that all men are his neighbours, and that not only the foreigner just beyond the frontier, but also the naked savage, chipping his stone axe or weaving his rude basket for the reception of his neighbour’s head, is a man of like passions with himself, with equal claims upon justice and freedom and all that makes the humanity of man. It only remains to point out the part in human life of a new factor of progress which promises to eclipse all others in importance. The main theme of my earlier work[153] was that only through increase of knowledge of others is each man’s knowledge of himself slowly built up and enriched, until it renders him capable of enlightened self-direction. So the main theme of this book is the development of the group mind, the increase of its self-knowledge and of its power of self-direction through increase of knowledge of other human societies.