Throughout his career Maeterlinck reveals himself in the double aspect of poet and philosopher. In the first period his philosophy, as has already been amply hinted, is characterized chiefly by aversion from the externalities of life, and by that tense introversion of the mind which forms the mystic's main avenue to the goal of knowledge. But if, in order to find the key to his tragedies and puppet plays, we go to the thirteen essays representing the earlier trend of his philosophy and issued in 1896 under the collective title, "The Treasure of the Humble," we discover easily that his cast of mysticism is very different from that of his philosophic predecessors and teachers in the fourteenth and nineteenth centuries, in particular from the devotional mysticism of the "Admirable" John Ruysbroeck, and Friedrich von Hardenberg-Novalis. Maeterlinck does not strive after the so-called "spiritual espousals," expounded by the "doctor ecstaticus," Ruysbroeck, in his celebrated treatise where Christ is symbolized as the divine groom and Human Nature as the bride glowing with desire for union with God. Maeterlinck feels too modernly to make use of that ancient sensuous imagery. The main thesis of his mystical belief is that there are divine forces dormant in human nature; how to arouse and release them, constitutes the paramount problem of human life. His doctrine is that a life not thus energized by its own latent divineness is, and must remain, humdrum and worthless. It will at once be noticed that such a doctrine harmonizes thoroughly with the romantic aspiration. Both mystic and romantic teach that, in the last resort, the battlefield of our fate lies not out in the wide world but that it is enclosed in the inner self, within the unknown quantity which we designate as our soul. The visible life, according to this modern prophet of mysticism, obeys the invisible; happiness and unhappiness flow exclusively from the inner sources.