The unified legal person emerged with property law and discipline.

Evidence

At least as far back as John Locke, “person” was theorized as a forensic term that called for the integration of a single subject with a continuous consciousness in a single body. The development of property law in a nascent capitalism was important to this conception. But equally important was the way attributing an essence to him helped the human subject to become an object of social discipline. — Talal Asad, Formations of the Secular, 357