The senses are the second constituent of human existence. Even if the body is healthy, the senses may be unhealthy. The senses will get diseased if they are overused. On the other hand, the senses will become useless if they are not used.
If the eyes are strained too much, they will become diseased. If we do not use the eyes at all, their efficiency will be reduced. The senses retain their efficiency so long as they are being properly used. The senses will be healthy when they are not overused or unused, but are properly used.