לזכות משה יהודה בן פעשא דינה
לרפואת נעמי בת טובה
It is well known that the Rambam didn't write the 13 Ani Maamin's printed in the siddur but rather they were written by the person who wrote them [we don't know who he was] BASED on the Rambam's 13 principles of faith. However a careful comparison yields many differences.
Principle number 3 is:
This implies that our belief is that Hashem has no body and that those with a body cannot possibly perceive of Him. He is beyond human scrutiny.
The Rambam himself, however, wrote in his Peirush Hamishnayos in delineating this principle:
"היא שנאמין כי האחד הזה שזכרנו אינו גוף ולא כח בגוף ולא ישיגוהו מאורעות הגופיים כמו התנועה והמנוחה והמשכן לא מצד עצמות ולא במקרה וכו".
And in Hilchos Yesodei Hatorah [1/11]
"וכיון שנתבהר שאינו גוף וגויה יתבאר שלא יארעו ולא אחד ממאורעות הגוף לא חיבור ולא פירוד וכו"
Meaning that we just must believe that Hashem has no bodily characteristics like movement and rest. But nothing about human beings not being able to perceive Him. It could be that the words ולא ישיגהו משיגי הגוף in fact means not that he is beyond human conception but that bodily things don't apply to him - but the words in the siddur are at best ambiguous.
And the words ואין לו שום דמיון כלל can lead one to believe that we can't possibly imagine Him. But in fact it means that he has nothing that resembles the body about Him. As we say in Yigdal [which is based on the 13 ikkarim] אין לו דמות הגוף ואינו גוף. [See Yesodei Hatorah 1/8].
However - it is ALSO true that we cannot possibly imagine who Hashem is, as the Rambam discusses in the Moreh Nevuchim [1/51 and on].
[Based on Yeshurun 22 page 797. See also Da'as Emunah - Ha-ikkar Hashlishi where he explands upon this principal]