What causes inharmony? The lack of (1) tone and (2) rhythm. How can it be interpreted in physical terminology?
(1) Prana, or life, or energy is the tone.
(2) Circulation, regularity is the rhythm -- regularity in the beatings of the head, of the pulse and the circulation of the blood through the veins.

In physical terms,
(2) the lack of circulation means congestion;
(1) and the lack of Prana, or life, or energy means weakness.
These two conditions attract illness and are the cause of illness.

In mental terms,
(2) The rhythm is the action of the mind, whether the mind is active in harmonious thoughts or in inharmonious thoughts, whether the mind is strong, firm, and steady, or whether it is weak. If one continues to think harmonious thoughts it is just like regular beating of the pulse and proper circulation of the blood; if the harmony of thought is broken, then the mind becomes congested. Then a person loses memory; depression comes as the result, and what one sees is nothing but darkness. Doubt, suspicion, distrust, and all manner of distress and despair come when the mind is congested in this way.
(1) The Prana of the mind is maintained when the mind can be steady in thoughts of harmony; then the mind can balance its thoughts, then it cannot be easily shaken, then doubt and confusion cannot easily overpower it.

Whether it is nervous illness, whether it is mental disorder, whether it is physical illness, at the root of all these different aspects of illness there is one cause, and that cause is illharmony.

The body which has once become inharmonious turns into a receptacle of inharmonious influences, of inharmonious atoms; it partakes of them without knowing it; and so it is with the mind. The body which is already lacking in health is more susceptible to illness than the body which is perfectly healthy; and so the mind which already has a disorder in it is more susceptible to every suggestion of disorder, and in this way goes from bad to worse.

Scientists of all ages have found that each element attracts the same element, and so it is natural that illness should attract illness; thus in plain words inharmony attracts inharmony, whereas harmony attracts harmony.

We see in everyday life that a person who has nothing the matter with him and is only weak physically, or whose life is not regular, is always susceptible to illness. Then we see that a person who ponders often upon inharmonious thoughts is very easily offended, it does not take long for him to get offended; a little thing here and there makes him feel irritated, because irritation is already there, it wants just a little touch to make it a deeper irritation.

Besides this the harmony of the body and the mind depends upon one's external life, the food one eats, the way one lives, the people one meets, the work one does, the climate in which one lives. There is no doubt that under the same conditions one person may be ill and another may be well. The reason is that one is in harmony with the food he eats, with the weather he lives in, with the people whom he meets, with the conditions around him. Another person revolts against the food he eats, against the people he meets, against the conditions that surround him, against the weather he must live in. This is because he is not in harmony; and he perceives and experiences similar results in all things in his life; disorder and illness are the result.

This idea can be very well demonstrated by the method that present-day physicians have adopted, of inoculating a person with the same element which makes him ill. There is no better demonstration of this idea than the practice of inoculation. This puts a person in harmony with the thing that is opposed to his nature. If one understands this principle one can inoculate oneself with all that does not agree with one, and with that to which one is continually exposed and from which there is no means of getting away. Woodcutters do not as a rule get sunstroke: seamen do not catch cold easily. The reason is that the former have made themselves sunproof while the latter have made themselves waterproof.

In short, the first lesson in health is the understanding of this principle, that illness is nothing but inharmony and that the secret of health lies in harmony.