Imam As-Sadiq (a.s.) said:
إحْذَرُوا اَهْوائَكُمْ كَما تَحْذَرُونَ أَعْدائَكُمْ فَلَيْسَ شَىْءٌ أَعْدى لِلرَّجالِ مِنْ إِتِّباعِ أَهْوائِهِمْ وَ حَصائِدِ اَلْسِنَتِهِمْ
Beware of sensual desires in the same way as you watch out for your (obstinate) enemies because man has no enemies greater than following sensual desires and the outcomes of his tongue!1
Undoubtedly, internal enemies are more dangerous than external foes. Therefore, obstinate desires which affect man inwardly, are considered as the most dangerous enemy for man. Sensuality blinds the eye and deafens the ear. It extinguishes the light of wisdom, distorts the face of truth and finally leads him to the precipice of corruption.