Sri Aurobindo
Letters of Sri Aurobindo
Volume IV - Part 3
You have only to admit that the mind and vital can influence the body – then no difficulty is left. In this action of mind and vital on the body faith and hope have an immense importance. I do not at all mean that they are omnipotent or infallibly effective 
– that is not so. But they assist the action of any force that can be applied, even of an apparently purely material force like medicine. In fact however there is no such thing as a purely material force, but the action may be purely material when it is a question of material objects. But in things that have life or mind and life one cannot isolate the material operation like that. There is always a play of other forces mixed with it in the reception at least and for the most part in the inception and direction also.