From Seven Objectives of Tefila (#864) by Rabbi Avigdor Miller
Now one of the best occasions to work on bitachon (trust in Hashem) is tefila (prayer). When you say “atah chonen l’adam daas — You bestow wisdom on man” (Shemoneh Esreh), why do you say atah chonen (You bestow)? You don’t start all the tefillos with an atah (You). You say Shema Koleinu (Hear our voice), Refaeinu (Heal us), Bareich Aleinu (Bless us). When it comes to daas (Knowledge), we say atah chonen.
People think, I have daas, my own daas. No, you don’t have any daas! It needs a hakdama (preface), and the hakdama is atah: You give daas. We don’t have any daas. Baruch Hashem every minute Hashem is giving you daas. The reason you’re not chalila in an insane asylum in a padded cell and raging and knocking your head against the wall is atah. You’re learning now that Hashem is the one who gives daas. He is the one who is making you sane... Hashem is giving you daas. It’s a gift He’s giving you right now. Therefore, you’re learning bitachon.
The reason you’re not in an insane asylum knocking your head against the wall is atah — Hashem
You say, “refaeinu Hashem v’neirafei — heal us, Hashem, and we will be healed.” Let a person realize how many hundreds of thousands of complicated situations take place in his body every day. Everyone is a miracle. You have in your brain very thin capillaries where the blood flows slowly, corpuscle by corpuscle. It’s so thin the blood corpuscles have to go in single file. Why doesn’t it happen that in the very thin tubes the blood gets stuck? If it gets stuck and clogged chas v’shalom in the brain — a stroke right away. But Hashem keeps the blood liquid, even those very fine capillaries, and it flows through. If a corpuscle will get stuck, the next one comes and nudges it, pushes it, and it clears it and you’re saved.
Then, “baruch atah Hashem — Blessed are you Hashem — rofei cholei amo Yisrael — Healer of His people Israel,” You Hashem are the one who keeps us well. You are healing us. Not merely when we’re not sick; You’re healing us constantly. So you are working on your bitachon. Of course when it’s necessary to take a refuah, we should take the medicine. But you have to know that Hashem is the one who heals us. Your mitzvah is to take the medicine. But work on that emunah that only Hashem is the rofei cholei amo Yisrael.