To render this still more plain, let us illustrate it by an example. Let us imagine a magic lantern capable of projecting living and corporeal images upon a living screen. External science occupies itself only with these images, the relations which they bear to each other and the changes taking place between them; but it knows nothing about the slides in the lantern upon which are the types of these visible images, and it entirely ignores the light which causes their projection upon that screen; but he who sees the slides with its pictures and knows the source of the light which produces these shadow pictures does not need to study the shadows for the purposes of drawing inferences and speculating in regard to their causes. Thus there is a superficial science which is at present the object of pride of the world, and a secret science of which next to nothing is publicly known, but which is known to the wise and revealed by one’s own perception of truth.[11] Truths must be perceived before they can be intellectually grasped, and therefore this greater and higher science cannot be learned in books, nor be taught in lectures at college, it is the result of a development of man’s higher perception, belonging to his higher nature, and characterises the born physician. Without this superior faculty, known in its initial stage as the power of “intuition,” a medical practitioner can find occupation only in the outer yard of the temple, picking up useful grains among the rubbish; but he cannot enter the temple in which nature herself teaches her divine mysteries. The minute details of this rubbish have been studied by modern popular science, whose attention has been so much absorbed thereby that the temple of truth itself has been forgotten and the nature of Life has become a mystery to those who only study its external manifestations. It will hardly be necessary to say that the above is not intended to discourage the study of phenomena; for those who have not the power of reaching higher will gain nothing by remaining ignorant of external appearances; but it is intended to show that a science referring merely to the phenomena of terrestrial life and ultimate results is not the summit of all possible knowledge; for beyond the realm of visible phenomena there is a far more extensive realm open to all who are capable of entering: the realm of truth, of which only the inverted images are seen in the kingdom of external phenomena. The natural science of the ancient mystics, owing to their deeper penetration into the so-called supersensual realm, was not limited to the world which we see with our bodily eyes; for they recognised four worlds or planes of existence within each other, each of them having its own forms of life and inhabitants, namely:— (_a._) _The physical visible world_, being only the reflection of the three higher ones. (_b._) _The astral world_, or the psychic realm. (_c._) _The world of mind_, or the spiritual realm.