Abstract
Aquinas accepts the definition of persona by Boethius, i.e., rationalis naturae individua substantia (individual substance of rational nature). But this definition causes some problems. Among them, the biggest problem is the tension between the substantiality and relationality of persona. Some modem interpreters criticize Aquinas for neglecting the relational aspect of the human person (persona humana). In this paper, I defend the position of Aquinas by paying attention to his analysis of the act of ingelligere (understanding). As a rational substance, persona humana can understand the external world and himself. But there exists an order here. He cannot understand himself immediately as a rational substance, but he understands himself by first understanding the external world. So, the self-understanding of the rational substance is mediated by the relationships it forms with the external world. Consequently, the substantiality of persona humana does not exclude the relationships with other things and persons. On the contrary, the substantiality of persona humana is perfected by these relationships. In this way, Aquinas succeeds in integrating the relational aspect and the substantial aspect of the human person in the activity of understanding.
