It's a question that confronts most of us in one form or another. A beginner may ask it. Or an experienced Christian Scientist may in some circumstances wonder about it in connection with his own healing work.
The reason the question is so prevalent is that it's the perennial question of a mind that believes it resides in matter. This may be less a real question than a habitual attitude. Mortal mind doesn't expect any answer, really, because from its limited viewpoint no one knows enough to heal on a spiritual basis.
The thousand and one doubts it dreams up as to whether one knows enough to heal, all have their basis in the mistaken belief that one's selfhood is an independent entity which lives and has its being in a matter world. This so-called "I" or entity is inevitably unaware of what God is and what God is doing. It lives with its own very dim view of spiritual reality, and therefore it frequently doubts whether it knows enough to heal.