שלשה אנשים נצבים עליו
“…Three men stood opposite [Avraham]…” 
Three men [who were really angels] — “One to notify Sarah of the birth of [Yitzhak/Isaac], one to destroy Sodom, and one to heal Avraham. For one angel doesn’t carry out two missions…But Raphael, the angel who healed Avraham, went from there to rescue Lot.” 
A story is told:
“On the very day that he assumed the Rabbinate of Brody, Galicia, the famous Rabbi Shlomo Kluger was asked to be sandak [godfather] at a bris [circumci-sion]. Arriving at the parents’ home, he learned that the child’s father was dying and that, according to a local minhag [custom], the bris would be deferred until after the father’s death, so that the infant could be given the father’s name. Rabbi Kluger quickly called a minyan and had the bris done at once.
To everyone’s amazement, the father spontaneously recovered!
The entire city was astir at the miracle that had happened.
Rabbi Kluger explained that he’d based his action on the ‘Rashi’ cited above:
‘Is there a lack of angels in heaven,’ he’d asked himself, ‘that the same angel sent to heal Avraham had to be sent also to rescue Lot?’
It seemed to him that the only explanation was: Lot’s merits hadn’t been enough to send an angel to rescue only him. So, the angel who healed Avraham was sent to help Lot, too, ‘on the way.’
‘It occurred to me,’ Rabbi Kluger said, ‘that the infant’s father was being judged in Heaven and that his merits hadn’t been enough for Eliyahu ha-Navi [the prophet Elijah] to come down to earth solely to bring him healing. But since Eliyahu attends every bris, I had it done at once, so that Elihayu might come down immediately, bringing healing to the child and father, too, ‘on the way’.” 
The Tzemach Tzedek, too, once told the father of a child who was near death: טראכט גוט וועט זיין גוט — Tracht gut vet zein gut; “Think positively, and the out-come will be good.” [lit.: “Think good and it’ll be good.”] The child healed. 
Rabbi Kluger, roughly contemporary with the Tzemach Tzedek, wasn’t a Hasidic teacher. [*] Yet, both were applying the same spiritual principle or “law,” expressed in the Zohar as:
It’a’ru’ta d’l’ta’ta, it’a’ru’ta d’l’e’la — “An awakening (or “push”) from below (creates) an awakening (or “push/response”) from above.” 
From this, we learn the essential principle of healing prayer:
G-d — present everywhere in and around us — responds in kind to the content of our belief.
As the Nefesh HaChaim said: “Just as G-d is Elokim, ‘Master of all forces’ everywhere, guiding and directing them each moment, so it [is] His Will to grant [us] sovereignty over countless forces and worlds through the way [we conduct ourselves] in [our] actions, words, and thoughts at every moment.” 
Everyone else in Brody believed the father was about to die. Rabbi Kluger did, too, for a moment. But he immediately replaced this with the undiluted belief that Eliyahu, attending the bris, would be bringing healing. He also believed without any doubt — based on Rashi’s comment on Ber./Gen. 18:2 — that an angel (in this case, Eliyahu) who brings help, can and will bring it to more than one person. It’s mission still remains only “one.”
In that instant, Rabbi Kluger’s own thinking changed. In his “mind’s eye,” he no longer saw the father dying. Based on his belief and reasoning, he could only see the father as healed.
As Rabbi Morris Lichtenstein later taught:
“In these mental prayers, there should never be formed any negative images…[one] should see always with his [or her] mental vision only the state in which he [or she] desires to be…” 
The change in Rabbi Kluger’s own thoughts brought about the healing.
Rabbi Kluger’s explanation also demonstrated to himself and everyone else that his belief was based in Torah — therefore, on a higher level of awareness of the Divine Presence and Involvement.
What’s more, the community’s minhag of deferring the bris was reinforcing a belief by everyone, including the father, that the father would die. Performing the bris immediately strongly impressed and affirmed for everyone the counter-thought that the father would instead continue to live and be well.
The change in the community-thought contributed to the healing.
Finally, that Rabbi Kluger saw this process as involving an angel or Eliyahu means that his change in thought was taking place not in the “rational” or “conscious” part of his mind, but in his imagination — which in its deeper levels borders the Infinite:
“…G-d cannot be perceived through the mind alone. If you would know G-d, do not seek merely to prove His existence, but turn to Him with your heart; affirm your union with Him, affirm His responsiveness to prayer, pray to Him; if you actually turn to G-d … speak to Him in your heart, you will be astonished to find how close He is to you, you will feel His nearness, you will have found G-d.” 
It’s not that “we” ever heal another or ourselves. Healing always comes from G-d. But we allow healing to occur by seeing only that perfect state — just as Rabbi Kluger did:
“When we pray with the imagination, when we visualize our prayer, when we see with our mind’s eye the state in which we wish to be, we are addressing our prayer to the Divine forces within ourselves; we are invoking them into action by the visualized declaration of that which we wish to attain.” 
That’s the principle: G-d is always responding in kind to our thoughts.
We apply it, then, by the thoughts that we intentionally choose in prayer.
And at all other times, too.