Step 1: Understanding the Concept:
The question asks to identify the philosopher who believed in mind-body dualism, the idea that the soul (or mind) is a non-physical substance that can exist separately from the physical body.
Step 2: Detailed Explanation:
(A) Plato: A classical Greek philosopher, Plato was a strong proponent of dualism. He argued that the soul is immortal and can exist independently of the body, both before birth and after death. His dialogue, the *Phaedo*, specifically presents arguments for the soul's immortality and its separateness from the body.
(B) De Anima: This is not a person but the title of a major work by Aristotle ("On the Soul"). Aristotle, Plato's student, had a different view. He believed the soul was the "form" of the body and could not exist without it, which is a view contrary to dualism.
(C) Rene Descartes: A 17th-century philosopher, Descartes is also a famous dualist. He famously argued, "I think, therefore I am," to establish the existence of the mind as a thinking thing, distinct from the extended, physical body. While Descartes is a correct example of a dualist, Plato's ideas on this topic predate his by centuries, and Plato is a foundational figure for this concept. Given the options, Plato is the most historically primary answer.
(D) Gustav Fechner: A 19th-century German psychologist, Fechner is known as a founder of psychophysics, the study of the relationship between physical stimuli and sensory experience. His work is not primarily focused on the metaphysical concept of the soul's separability from the body.
Step 3: Final Answer:
Based on the foundational and explicit arguments for the soul's separate existence, Plato is the correct answer.