Islamic economics is a unique system that differs from other economic systems in many ways. Its defining characteristic can be said to be the prohibition of interest. Unfair means of earning are forbidden in Islam to ensure social justice. The prohibition of interest is motivated by the fact that it represents an unfair means of earning. According to the statements of scholars of Islamic legal theory (uṣūl), all legal provisions are justified by attracting benefits (jalb al-manāfi‘) and avoiding harms (daf‘ al-mafāsid). There are benefits and reasons that can be comprehended by the human mind in the prohibition of interest. Foremost among them is the principle of preserving wealth, which is one of the five universal principles that Islam aims to achieve. Islam unequivocally prohibits all forms of unfair means of earning in order to preserve wealth. In this regard, the definition in modern economic thought that describes interest as the rental price of capital or the price paid for the deprivation of the capital owner from his/her capital for a certain period is highly problematic from an Islamic perspective. Similarly, some scholars consider interest as contrary to creation. Many scholars and economists also argue that interest has financial, moral, and social damages. This study firstly mentions the definition of interest and its causes briefly, and then evaluates the issue with regards to maqāṣıd and ḥikmat al-tashrī‘.