Why did St. Thomas Aquinas believe the human soul is immortal?

St. Thomas Aquinas argued for the immortality of the human soul by synthesizing Aristotelian philosophy with Christian theology. While Aristotle generally viewed the "form" of an object as inseparable from its matter, Aquinas posited that the human soul is a unique "substantial form".12

Aquinas maintained that because the soul is a spiritual, self-subsisting entity that possesses no matter and contains no substantial parts, it cannot be corrupted or disintegrated upon the death of the body. He argued that the soul is an indissoluble unity capable of existing independently of matter. Consequently, he believed that the immortality of the human soul could be strictly demonstrated through reason, as it functions as a subsistent form that remains intact even when separated from the physical body. This perspective allowed Aquinas to affirm individual immortality, distinguishing his views from those who argued that the soul was merely a material entity or that the intellect was reabsorbed into a universal consciousness after death.3124