I guess you are thinking of this verse:
“Know thou that the soul of man is exalted above, and is independent of all infirmities of body or mind. That a sick person showeth signs of weakness is due to the hindrances that interpose themselves between his soul and his body, for the soul itself remaineth unaffected by any bodily ailments.” Gleanings From the Writings of Bahá’u’lláh, p. 153-154
The entire passage is on this link:
Gleanings From the Writings of Bahá’u’lláh, LXXX
I wrote up my own explanation of the passage because people have asked me about it.
Every human being has a soul. The soul is what animates the brain and allows the brain to function, but the soul has to work through the brain while we are live in a physical body in order for us to be able to think intelligently.
Those who have brain dysfunctions still have a soul but the soul is hindered from fully expressing itself because the brain damage interferes with the soul’s functioning. So in the case of Alzheimer’s disease we cannot see the personality of the person express itself, but it is still there, since the soul is the person himself, thus the personality. When that person dies and his soul is freed from the physical body, he will no longer be affected by that brain damage since he will no longer have a brain to hinder the soul from functioning, so the soul will be free to express itself and the personality will be seen.
If someone did not believe there was a soul, they might think that an Alzheimer’s patient had no personality, but the Baha’i belief is that the soul is hidden as if under a bushel unable to express itself, because of the brain disease which interposes itself in between the soul and the body.
To recap what I said before, your soul is responsible for consciousness, but while you are alive in a body, your soul works through the brain. The soul communicates its desires through the brain to the physical body, which thereby expresses itself in various ways. The soul is responsible for the mind, senses and emotions as well as physical sensations, but these are expressed through the body. When the brain is damaged hindrances interpose themselves between the soul and the body and cause physical illness. The body is affected but the soul remains unaffected.
The soul is like the light of the lamp. An external object may interfere with its brightness, but the light itself continues to shine. Or think about the soul as the sun above the clouds. The clouds are simply preventing us from seeing the sunshine, but the sun is shining just as brightly nevertheless.
Every illness afflicting the body of man is an impediment that prevents the soul from manifesting its inherent might and power. When the soul leaves the body, however, it will be unaffected by any bodily ailments and it will be able to fully manifest its power.