"And when he completed his interpretation he raised his eyes on high with mighty devotion, blessings and thanks to His great Name, may it be blessed, Who enabled him to conceive the light of all the Torah from within and from without. Thus he said, All the wisdoms are needed for our holy Torah and are included within it, and he mastered all of these perfectly. And he recalled all of them: the wisdom of algebra and triangles and geometry and the wisdom of music...and of the wisdom of philosophy, [The Gaon] said he knew it perfectly. And he brought only two good things out of it...and the rest must be thrown out. Then he said, thank God the entire Torah, which was given at Sinai, he knew it thoroughly, and all of the Prophets and Writings and mishnayot and the oral law, how they are hidden in it, and no doubt was left to him about any Halakhah or sugiya in the whole Torah in his old age, and he knew the entire oral Torah and all the Halakhic authorities up to the recent ones on the Shulhan Arukh, and he clarified them and shined light onto darkness of flawed readings;...and in hidden things all that was in our possession, the Zohars and the Tiqunei Zohar and Sefer Yezira and the writings of the ARI of sacred memory and the PARDES, he studied them and knew them...And he revised them with evidence as clear as sun; only two grave things in the mysteries of the Torah of the Zohar were questionable to him...And those, if he knew who knew them, he would go on foot to him and then wait for our righteous messiah, and with this he finished."
The Gaon of Vilna: The Man and His Image, by Immanuel Etkes; Quoting Hakdama to Pe'at HaShulkhan by Rav Yisrael of Shklov.