Final answer:
The twelfth-century heretics known as dualists believed in two separate substances that make up reality: the material or physical, and the mental or spiritual, a view notably supported by René Descartes. Their belief indicates a clear division between the body and the mind, with the notion that the self is primarily a thinking, non-extended entity.
Step-by-step explanation:
The twelfth-century heretics known as duelists believed in a form of metaphysical dualism which posits that there are two fundamental kinds of substance that make up reality: the material or physical, and the spiritual or mental. This idea suggests a clear separation between the body, seen as an extended entity in space and time (res extensa), and the mind, seen as a thinking, non-extended entity (res cogitans).
This concept was famously championed by René Descartes, who articulated the view that the essence of the self is a thinking thing (res cogitans), inherently separate and potentially immortal, existing independently from the physical body.
Descartes' substance dualism has been widely discussed and criticized, particularly given modern scientific understanding of the brain and consciousness. Nevertheless, the idea of dualism has significantly influenced Western thought, especially within the Judaeo-Christian-Islamic traditions, which often reinforce a similar division between the material and the spiritual realms. While Descartes provides a sharp separation of mind and body, other philosophers like Spinoza proposed alternative models, such as the double-aspect theory, which suggests that the mental and physical are two aspects of a single substance, which he identified as God.