The theory of substance and accident, which was inherited from Aristotle, is one of the most significant topics in the history of classical philosophy. It has significant implications in both the physical and metaphysical fields. Its influence in these fields is not merely an extension arising in the context of different problems; rather, it is one of the most significant theories in the development of physics and metaphysics. Furthermore, the theory of substance and accident is also a key subject in logic. It is particularly relevant in the context of categories. Both philosophers and theologians in Islamic thought have used this theory since its inception, and its application and sphere of influence have differed between the two schools. They differ significantly on sub-issues such as the understanding and definition of substance, the priority of accident over substance, the existence of accident in accident, and the designation of God as substance. Although the essence-attribute theory dates back to Aristotle, the meaning of this concept in his work is unclear. This ambiguity has preoccupied subsequent philosophers. In the works of Ibn Sina, one of the Islamic world’s leading philosophers, the theory is seen to have gained a clearer and more explicit meaning through commentators’ solutions to the problems raised by this concept. This study examines Ibn Sina’s definition of substance and how he used the concept, as well as his contributions to existing definitions and classifications of substances. It highlights areas in which the philosopher’s views differed from those of Aristotle. In this context, Ibn Sina’s work Al-Shifa: Metaphysics and al-Shifa: Categories, alongside Aristotle’s Metaphysics and Categories, became the primary sources of reference. References were occasionally made to the similarities and differences between Ibn Sina’s views on various topics and issues, and the views of certain Aristotelian commentators, such as Simplicius, and philosophers, such as Plotinus and Porphyry. The aim is to demonstrate that the phrase ‘when found in a’yan’ added to the definition of substance is related to understanding substance in terms of its essence, as well as correctly understanding and applying the classification into first and second substances. It is also demonstrated that the concept of ‘never’ added to the phrase ‘not found in a subject’ in the definition of substance refers to the immutability of substance’s nature and the unacceptability of what is found as substance in one place being found as an accident in another. The Aristotelian distinction between first and second substances is problematic because it prioritises the sensory. To overcome this, a distinction is made between ‘priority’ and ‘worthiness’. Furthermore, it is demonstrated that, in Ibn Sina’s understanding of substance, priority is given to essences and rational substances.