P15: The idea that constitutes the formal being {esse} of the human Mind is not simple, but composed of a great many ideas.

Dem.: The idea that constitutes the formal being of the human Mind is the idea of a body (by P13), which (by Post. 1) is composed of a great many highly composite Individuals.

But of each Individual composing the body, there is necessarily (by P8 C) an idea in God/Nature. Therefore (by P7), the idea of the human Body is composed of these many ideas of the parts composing the Body, q.e.d.