Certainly, one can act from within on an illness and cure it. Only it is not always easy as there is much resistance in Matter, a resistance of inertia. An untiring persistence is necessary; at first one may fail altogether or the symptoms increase, but gradually the control of the body or of a particular illness becomes stronger. Again, to cure an occasional attack of illness by inner means is comparatively easy, to make the body immune from it in future is more difficult. A chronic malady is harder to deal with, more reluctant to disappear entirely than an occasional disturbance of the body. So long as the control of the body is imperfect, there are all these and other imperfections and difficulties in the use of the inner force.

If you can succeed by the inner action in preventing increase, even that is something; you have then by abhyasa to strengthen the power till it becomes able to cure. Note that so long as the power is not entirely there, some aid of physical means need not be altogether rejected.

Medicines are a pis aller that have to be used when something in the consciousness does not respond or responds superficially to the Force. Very often it is some part of the material consciousness that is unreceptive—at other times it is the subconscient which stands in the way even when the whole waking mind, life, physical consent to the liberating influence. If the subconscient also answers, then even a slight touch of the Force can not only cure the particular illness but make that form or kind of illness practically impossible hereafter.

Your theory of illness is rather a perilous creed—for illness is a thing to be eliminated, not accepted or enjoyed. There is something in the being that enjoys illness, it is possible even to turn the pains of illness like any other pain into a form of pleasure; for pain and pleasure are both of them degradations of an original Ananda and can be reduced into the terms of each other or else sublimated into their original principle of Ananda. It is true also that one must be able to bear illness with calm, equanimity, endurance, even recognition of it, since it has come, as something that had to be passed through in the course of experience. But to accept and enjoy it means to help it to last and that will not do; for illness is a deformation of the physical nature just as lust, anger, jealousy, etc., are deformations of the vital nature and error and prejudice and indulgence of falsehood are deformations of the mental nature. All these things have to be eliminated and rejection is the first condition of their disappearance while acceptance has a contrary effect altogether.

All illnesses pass through the nervous or vital-physical sheath of the subtle consciousness and subtle body before they enter the physical. If one is conscious of the subtle body or with the subtle consciousness, one can stop an illness on its way and prevent it from entering the physical body. But it may have come without one's noticing, or when one is asleep or through the subconscient, or in a sudden rush when one is off one's guard; then there is nothing to do but to fight it out from a hold already gained on the body. Self-defence by these inner means may become so strong that the body becomes practically immune as many yogis are. Still this “practically” does not mean “absolutely”. The absolute immunity can only come with the supramental change. For below the supramental it is the result of an action of a Force among many forces and can be disturbed by a disruption of the equilibrium established—in the supramental it is a law of the nature; in a supramentalised body immunity from illness would be automatic, inherent in its new nature.

There is a difference between yogic Force on the mental and inferior planes and the supramental Nature. What is acquired and held by the yoga-Force in the mind-and-body consciousness is in the supramental inherent and exists not by achievement but by nature—it is self-existent and absolute.