If there is prolonged thought upon a subject this increase becomes permanent, and it is thus open to any man to increase the size of his mental body either along desirable or undesirable lines.
Good thoughts produce vibrations of the finer matter of the body, which by its specific gravity tends to float in the upper part of the ovoid; whereas bad thoughts, such as selfishness and avarice, are always oscillations of the grosser matter, which tends to gravitate towards the lower part of the ovoid.

Consequently the ordinary man, who yields himself not infrequently to selfish thoughts of various kinds, usually expands the lower part of his mental body, and presents roughly the appearance of an egg with its larger end downwards.

The man who has repressed those lower thoughts, and devoted himself to higher ones, tends to expand the upper part of his mental body, and therefore presents the appearance of an egg standing on its smaller end.

From a study of the colours and striations of a man's mental body the clairvoyant can perceive his character and the progress he has made in his present life.

From similar features of the causal body he can see what progress the ego has made since its original formation, when the man left the animal kingdom.
When a man thinks of any concrete object--a book, a house, a landscape--he builds a tiny image of the object in the matter of his mental body.