Shaar HaBitachon: Lesson 23

One can do nothing to increase the amount of wealth that G-d has already allotted for him. Likewise, a healing only works because G-d wants it to. Sometimes a livelihood and healing come from a place other than where one was looking for them. We must pursue natural means in order to conceal the miracle of G-d providing wealth and healing but the natural means do not actually have power to help or hurt us.

(Chapter 4)