I have heard that Sha'ar Ha'Yichud of Chovos Ha'Levavos is controversial, and that one needs a rebbe in order to learn it. What does the author say that is so difficult to understand?
The philosophical approach is considered by many to be an "Emunah minefield" for those who do not have proper guidance. The popular "Lev Tov" edition of Chovos Halevavos (with a translation/commentary by R' Pinchas Lieberman) has a lengthy introduction to Shaar HaYichud, in which he cites these views at length and in detail.
I asked this question to Rabbi Mordechai Kornfeld, the rosh kollel of iyun hadaf in harnof http://dafyomi.co.il
he replied: "There is a difference of opinion among the Torah authorities as to whether this section should be studied by the typical student of the Torah. Many Torah luminaries maintain that one should not seek philosophical proofs of G-d's existence. Belief in Hash-m should be based on the Mesorah (tradition) that we received from our elders and mentors, the study of Hash-m's wondrous Torah, and the many ways He manifests Himself in His creations and in our daily lives."
(after more study of the shaar yichud)
ray: The reason the shaar yichud is so difficult is because the author tries to explain logically why we exist. ultimately this involves trying to understand that which is Eternal, and the Eternal is beyond the grasp of human logic, since it is beyond our cause/effect way of understanding things. the shaar yichud itself concludes that it is better to stay away from logical inquiry and to try to know God through His deeds as written in ch.10:
Therefore, you should exert your mind until you know the Creator through the evidences of His works and not strive to know Him in His glorious essence. For He is exceedingly close to you from the side of His deeds but infinitely remote in any representation of His essence or comparison with it. As already stated, we will never be able to find Him in this way. When you arrive at the stage where you abandon (trying to find Him) through your thoughts and senses because He cannot be grasped in this way, and you instead find Him in the evidence of His deeds, as though He were inseparable from you - this is the pinnacle of knowledge of Him which the prophet exhorts us on.
from Rabbi Mattityahu Solomon's recently published commentary to chovos halevavos (gate 2):
"The custom practiced in the yeshiva world is not to study the Shaar Yichud. And even though, there is no doubt whatsoever that all of what he says there is absolute truth, nevertheless, his words are of philosophical inquiry and this inherently leads to many questions in the mind of the person studying them, and not every person is capable of fully understanding them. It is possible therefore that one could remain with unresolved questions, or at least with doubts, that would not have occured to him had he not studied this work. Therefore, it is customary to walk simply and accept as a given, simple faith that the Creator is One. And the explanation of One is that there is no power in the world besides Him, no place in the world devoid of Him, and nothing in the world without Him. These things are above the powers of our minds to grasp.
In addition to the issue of whether the philosophical proof of G-d's existence is a proper approach, my own analysis of Sha'ar HaYichud is that the particular philosophical proof he uses is simply incorrect. Some mathematical premises that he relies on were proved incorrect in the 19th century.
There is a general trend away from studying the Aristotelian Rishonim. Few Yeshivas study the Rambam's Guide for the Perplexed. And when studying his code, it is typical in those communities to start with the 5th chapter, skipping the metaphysics and physics in Yesodei haTorah (Fundamentals of the Torah) ch. 1-4.
This has little to do with rationalism vs anti-intellectualism. The Kuzari too was a philosopher in our sense of the word; when he denigrates philosophy he is talking about Scholasticism -- trying to make one truth out of Aristotelian and neo-Platonic notions on the one hand and one's religion on the other.
Because people today aren't bothered by Aristotle's questions, and the field of philosophy moved well beyond his answers, there is an argument that there is little reason to study versions of Judaism cast into those jugs. It just raises questions and doubts to ideas in Judaism one wouldn't otherwise be questioning.
Whereas the Kuzari's approach is more consistent with the West's thought since Kant, and therefore more useful in addressing our own questions as residents in that world. There is a reason why thinkers as diverse as Rav Dessler, Rav Hutner, Rav AY Kook and Rabbi JB Soloveitchik were all Kantian or neo-Kantian (and in R' Hutner's and RJBS's cases -- perhaps with touches of Existentialism). All very intellectual, and aside from R' Kook, very rationalist. But not Scholastic.
And so, the Noda biYhudah (Mahadura Qama OC 35) saw little reason to open up Shaar haYichud in Chovos haLvavos.
And the Gra counted philosophy as one of the "Seven Wisdoms" symbolized by the menorah, wished he would have the place in the World to Come enjoyed by the Rambam, but advises against the study of the aforementioned writings, products of his "being led astray by that הפילוסופיה הארורה -- accursed Scholasticism" (commentary on Shulchan Aruch, YD 179:13; with my translating "philosophia" as "Scholasticism").
(Personal note: I do not believe any of the above; I'm the kind of guy who paused my study of the Guide of the Perplexed early on so that I could read up on Aristotle, Plato and Plotinus to have the necessary background to understand Rav Saadia Gaon, the Rambam, or Rabbeinu Bachya.)