The focus of this paper is a first-century pseudepigraphic treatise titled On Human Nature, preserved by Stobaeus and attributed to the Pythagorean Aesara. Whether the treatise is to be ascribed to a woman philosopher named Aesara or the Pythagorean man Aresas is a point of controversy. In what follows, we gloss over the question of the identity and gender of the author and turn to the philosophical content of the treatise. In the surviving fragment, Aesara analyses the structure of the human soul and the relationships among its parts. The human soul becomes a model of law and justice for both the city and the household. Thus, On Human Nature revisits well-known Platonic doctrines, such as the city-soul analogy, the tripartition of the soul and the definition of justice as harmony, providing novel insights into their political implications. In what follows, we argue that Aesara constructs an original psychological theory by supplementing Plato’s tripartite conception of the soul in the Republic with aspects from later dialogues. Specifically, Aesara employs the Laws to stress the political implications of the tripartition and the leading role of νοῦς, and the Timaeus to explain our psychological reactions through physiological phenomena.