Sickness of the limbs!
Know that every limb was created to perform a specific action, so a sign of its [i.e. any limb] sickness is when guilt is established due to performing [an evil] act. A sign of the hand’s sickness is when it is guilty of striking unlawfully; a sign of eye’s sickness is when it is guilty of staring unlawfully; a sign of the heart’s sickness is when it is guilty of not doing what it was created for, such as acquiring knowledge and wisdom; love of Allaah, worshiping Allaah alone and preferring that over every other desire. Therefore, if a person was to possess knowledge of everything but he does not know Allaah (Gloried Be He), then it is as if he knew nothing. The sign of knowing Allaah is to love Allaah, for whoever truly knows Allaah will love Allaah, and a sign of having love for Allaah is that one does not give precedence to anything over Allaah. So whoever prefers something from the beloved things over Allaah, then his heart is sick, just like the stomach that prefers mud and not bread, due to losing the desire for bread. [Source: Mukhtasar Minhaaj Al-Qaasideen’ by Imaam Ibn Qudaamah Al-Maqdisee (rahimahullaah) page 164; Abridged and paraphrased]