"Περί Ψυχής" by Aristotle is a treatise written around 350 BC. It explores what makes living things alive, examining the different capacities of plants, animals, and humans. Aristotle argues that the soul is not separate from the body but rather its essence and form. He investigates nutrition, sensation, movement, and human intellect, proposing that the soul's nature can be understood through studying its operations and powers across different forms of life.