Just as one can suffer from diseases of the heart, so too can one suffer from diseases of the intellect. But in Islamic thought the heart and the intellect are intimately linked and inseparable such that they have a direct impact on each other.
Arrogance, for example, is typically understood as a disease of the heart, but arrogance also affects one’s intellect, i.e., one’s ability to understand and grasp truth. Arrogance can even affect the eyes such that one is blinded from truths that are right before one’s eyes. And this is an everyday thing that we can observe with people around us, where we wonder why some cannot see something so obvious.
Same goes for maladies such as jealousy, malicious hate, and greed. Such diseases are so severe that they not only lay waste to the soul but also infect the entire body, up to the point of clouding one’s ability to think clearly.
When you analyze the work of some Muslim reformers, you can see the telltale signs of these diseases in the arguments they make and the conclusions they draw. May Allah spare us from such a condition.