The first two verses in this octave make indirectly reference to Dante, Paradiso 13: 120. It is the old intellectualistic illusion - then formalized by Descartes, and often followed still nowadays - that the mind can understand things perfectly provided that feelings, emotions, etc., don't interfere. That implies a misunderstanding of both mind and feelings. On the other hand, these words are here put in the mouth of Aletes, who is trying to deceive Godfrey.
But the words about Fortune, though spoken by the same Aletes, are surely agreed on by Tasso himself. Indeed, they express the - probably - main conviction of Renaissance Man, whatever party he may belong to. Here also a quote from Dante is likely, see Inferno 7: 67-96, but it was anyway the subject of endless texts and works of art during the Middle Ages and Renaissance.
Dante, following Severinus Boethius, subjugates Fortune to God's Providence by changing it, or rather her, into an angel. Ariosto, on the contrary, shapes the whole plot of his Orlando Furioso according to the rules of the wheel of Fortune, even identifying God and chance on one occasion. Tasso tries to retrieve Dante's optimism, but he can't, both because of his own temper and because of the cultural milieu that had meanwhile developed. So, in Gerusalemme Conquistata he wonders, at first, whether Fortune is the influence of the stars, or simply chance, or a devil; and he finally creates a devil called Fortune.