Levi ben Gershon, or Gersonides as he was known in more latinized circles, or the Ralbag as he is known in more Jewish circles lived in the beginning of the 14th century. He was a great astronomer/astrologer (there's a crater named after him on the Moon), an inventor, a halakhist, Torah commentator, and a philosopher. As a philosopher, he was one in a long line of Jewish Rationalists who met Aristotle's well known teachings and attempted to integrate them into traditional Jewish theology. Maimonides is the most famous of these Jewish Rationalists.
What is interesting about Gersonides, however, is that although he is a godol in Halacha and Torah commentator, his religious views were often highly unorthodox. When rationalism would grind against what the Torah said he would, as opposed to the methodology of Maimonides, find in favor of rationalism.
"The position of Levi ben Gershon in Jewish philosophy is unique. Of all the Jewish Peripatetics he alone dared to vindicate the Aristotelian system in its integrity, regardless of the conflict existing between some of its doctrines and the principal dogmas of Judaism. Possessed of a highly developed critical sense, Levi sometimes disagrees with Aristotle and asserts his own views in opposition to those of his master, Averroes (Islamic Rationalist); but when, after having weighed the pros and cons of adoctrine, he believes it to be sound, he is not afraid to profess it, even when it is directly at variance with an accepted dogma of Jewish theology. "The Law," he says, "can not prevent us from considering to be true that which our reason urges us to believe" (Introduction to the "Milchamot," p. 6)."
I've highlighted a few of his more unorthodox views:
On Omniscience: "In contrast to the theology held by Orthodox Judaism, Gersonides held with those who denied God's omnipotence. "Gersonides, bothered by the old question of how God's foreknowledge is compatible with human freedom, holds that what God knows beforehand is all the choices open to each individual. God does not know, however, which choice the individual, in his freedom, will make." (Louis Jacobs, God, Torah, Israel: Traditionalism without Fundamentalism)"
"The sublime thought of God, he says, embraces all the cosmic laws which regulate the evolutions of nature, the general influences exercised by the celestial bodies on the sublunary world, and the specific essences with which matter is invested; but sublunary events, the multifarious details of the phenomenal world, are hidden from His spirit."
On the soul and immortality: "Gersonides posits that people's souls are composed of two parts: a material, or human, intellect; and an acquired, or agent, intellect.. The material intellect is inherent in every person, and gives people the capacity to understand and learn. This material intellect is mortal, and dies with the body. However, he also posits that the soul also has an acquired intellect. This survives death, and can contain the accumulated knowledge that the person acquired during their lifetime. For Gersonides, Seymour Feldman points out, "Man is immortal insofar as he attains the intellectual perfection that is open to him. This means that man becomes immortal only if and to the extent that he acquires knowledge of what he can in principle know, e.g. mathematics and the natural sciences. This knowledge survives his bodily death and constitutes his immortality." (Gersonides, Trans. Seymour Feldman Wars of the Lord, Book 1, p. 81, JPS, 1984)"
On Creation: "Having thus demonstrated that the world is not eternal "a parte ante" and is eternal "a parte post," Levi gives his own view of creation. He chooses a middle position between the theory of the existence of a primordial cosmic substance and that of a creation "ex nihilo," both of which he criticizes. According to him, there existed from eternity inert undetermined matter, devoid of form and attribute. At a given moment God bestowed upon this matter (which till then had only a potential existence) essence, form, motion, and life; and from it proceeded all sublunary beings and all heavenly substances, with the exception of the separated intelligences, which were direct emanations of the Divinity."
However, given such unusual views, it is to be expected that he would be criticized by the more traditional rabbis:
"Levi's philosophical theories, some of which influenced Spinoza (comp. "Theologico-Politicus," ch. ii., where Spinoza uses Levi's own terms in treating of miracles), met with great opposition among the Jews. While Hasdai Crescas criticized them on philosophical grounds, others attacked them merely because they were not in keeping with the ideas of orthodoxy. Isaac ben Sheshet (Responsa, No. 45), while expressing admiration for Levi's great Talmudical knowledge, censures his philosophical ideas, which he considers to be heresies the mere listening to which is sinful in the eyes of a pious Jew. Abravanel (commentary on Josh. x.) blames Levi in the harshest terms for having been so outspoken in his heretical ideas. Some zealous rabbis went so far as to forbid the study of Levi's Bible commentaries. Among these were Messer Leon Judah and Judah Muscato; the latter, applying to them Num. i. 49, says: "Only thou shalt not number the tribe of Levi, neither bring his Commentaries among the children of Israel" (Commentary on the "Cuzari," p. 4). Shem-Tob perverted the title "Milchamot Adonai" (= "Wars of God") into "Milchamot 'im Adonai" (= "Wars with God"); and by this corrupted title Levi's work is quoted by Isaac Arama and by Manasseh ben Israel, who attack it in most violent terms."
Despite all of these attacks however, the Ralbag is still found as a commentator in most printed Chumashim that have more than just Rashi and Onkelous. (Maimonides too had his fair share of critics and he too is found right along the great Torah commentators with the Ralbag.) And in modern strains of Orthodoxy, no Rishon can ever be "wrong." If a view was held by a Rishon, then that is an acceptable view in Orthodox Judaism.
What then does it mean to be "Orthodox" when there are so many options to believe? It is certainly apparent that Gersonides' views don't fit well with a number of Maimonides' "essential" thirteen ikkarim.
Sources: Jewish Encyclopedia and Wikipedia