Rambam- Yisdoei HaTorah Perek 7 Halacha 2: What Happens to a Prophet During Prophecy

הנביאים מעלות מעלות הן. כמו שיש בחכמה חכם גדול מחבירו כך בנבואה נביא גדול מנביא. וכולן אין רואין מראה הנבואה אלא בחלום בחזיון לילה. או ביום אחר שתפול עליהן תרדמה כמו שנאמר במראה אליו אתודע בחלום אדבר בו. וכולן כשמתנבאים אבריהן מזדעזעין וכח הגוף כשל ועשתנותיהם מתטרפות ותשאר הדעת פנויה להבין מה שתראה. כמו שנאמר באברהם והנה אימה חשכה גדולה נופלת עליו. וכמו שנאמר בדניאל והודי נהפך עלי למשחית ולא עצרתי כח: 

There are different levels of Prophets. This is similar to wisdom, [some people] have more wisdom than their friends, so too by prophecy some prophets are greater than others. [However, prophets on any level] only see prophecy in a dream, in a vision at night, or during the day after sleep has fallen on them like it says (Bamidbar 12:6), "I make Myself known in a vision, In a dream I speak with him." Anyone, when they are [overcome] with prophecy, their limbs shake, the power of their body fails, their thoughts become scrambled and the rest of their knowledge is free to understand what they are seeing (aka the prophecy) as it says by Avraham (Breishis 15:12), "Fear, a great darkness, fell on him." And like it says by Daniel (Daniel 10:8), "My glory was turned into corruptness and there remained no strength in me."      

The Rambam is describing what happens to prophets when they attain prophecy. The most interesting point made here is that prophecy occurs the same way in every prophet no matter how great the prophet or prophecy (The obvious exception here is Moshe who, in fact, communicated to G-D whenever he wanted). The Rambam bases this halacha on how the Torah, Prophets or Writings (Torah, Nevi'im, Kesuvim) describe a prophets actions or thoughts during a prophecy.

In the last halacha we mentioned that even if a person attains all the necessary preconditions G-D still must choose to give that person a prophecy. Here, we see that a supernatural event occurs during prophecy, G-D clears the mind of the prophet so that the prophet can focus all of his or her mental energy on the prophecy. If G-D did not clear the mind of the prophet then the prophet would be incapable of understanding the prophecy. Hence, a prophet must reach a high level of knowledge even to be considered for prophecy, but that does not mean that person will be granted understandable prophecy until G-D intervenes.