That secret portion of Freemasonry which is known only to the initiates as distinguished from Esoteric Freemasonry, or monitorial, which is accessible to all who choose to read the manuals and published works of the Order.
The words are from the Greek, meaning, internal, and, external, and were first used by Pythagoras, whose philosophy was divided into the exoteric, or that taught to all, and the esoteric, or that taught to a select few; and thus his disciples were divided into two classes, according to the Degree of initiation to which they had attained, as being either fully admitted into the society, and invested with all the knowledge that the Master could communicate, or as merely postulants, enjoying only the public instructions of the school, and awaiting the gradual reception of further knowledge. This double mode of instruction was borrowed by Pythagoras from the Egyptian priests, whose theology was of two kinds-the one exoteric, and addressed to the people in general; the other esoteric, and confined to a select number of the priests and to those who possessed, or were to possess, the regal power.
And the mystical nature of this concealed doctrine was expressed in their symbolic language by the images of sphinxes placed at the entrance of their temples. Two centuries later, Aristotle adopted the system of Pythagoras, and, in the Lyceum at Athens, delivered in the morning to his select disciples his subtle and concealed doctrines concerning God, Nature, and Life, and in the evening lectures on more elementary subjects to a promiscuous audience. These different lectures he called his Morning and his Evening Walk.