The great Second Hermetic Principle embodies the truth that there is a harmony, agreement, and correspondence between the several planes of Manifestation, Life and Being. This truth is a truth because all that is included in the Universe emanates from the same source, and the same laws, principles, and characteristics apply to each unit, or combination of units, of activity, as each manifests its own phenomena upon its own plane. For the purpose of convenience of thought and study, the Hermetic Philosophy considers that the Universe may be divided into three great classes of phenomena, known as the Three Great Planes, namely: 1. The Great Physical Plane. 2. The Great Mental Plane. 3. The Great Spiritual Plane. These divisions are more or less artificial and arbitrary, for the truth is that all of the three divisions are but ascending degrees of the great scale of Life, the lowest point of which is undifferentiated Matter, and the highest point that of Spirit. And, moreover, the different Planes shade into each other, so that no hard and fast division may be made between the higher phenomena of the Physical and the lower of the Mental; or between the higher of the Mental and the lower of the Physical. In short, the Three Great Planes may be regarded as three great groups of degrees of Life Manifestation. While the purposes of this little book do not allow us to enter into an extended discussion of, or explanation of, the subject of these different planes, still we think it well to give a general description of the same at this point. At the beginning we may as well consider the question so often asked by the neophyte, who desires to be informed regarding the meaning of the word "Plane", which term has been very freely used, and very poorly explained, in many recent works upon the subject of occultism. The question is generally about as follows: "Is a Plane a place having dimensions, or is it merely a condition or state?" We answer: "No, not a place, nor ordinary dimension of space; and yet more than a state or condition. It may be considered as a state or condition, and yet the state or condition is a degree of dimension, in a scale subject to measurement." Somewhat paradoxical, is it not? But let us examine the matter. A "dimension," you know, is "a measure in a straight line, relating to measure," etc. The ordinary dimensions of space are length, breadth, and height, or perhaps length, breadth, height, thickness or circumference. But there is another dimension of "created things" or "measure in a straight line," known to occultists, and to scientists as well, although the latter have not as yet applied the term "dimension" to it--and this new dimension, which, by the way, is the much speculated -about "Fourth Dimension," is the standard used in determining the degrees or "planes."