How have philosophers from Descartes to Kant understood the soul?

Question

Modern philosophy, beginning with Descartes, raises issues of mind-body dualism, rationalism and empiricism, idealism and materialism. Yet most modern philosophers at least through Kant continued to believe in the basically Christian concept of an immortal, "pre-psychological" soul, presumeably subject to duties and divine judgment.

How did Descartes, for example, define the soul within his dualism of res extension and res cogitans? Is is a distinct "substance" or "property"? Is it continuous with the being of God and with other souls...or a discrete, isolated entity? How does the soul relate to mind and/or body? How does it transform physical or mental activities into a timeless moral account?

Did subsequent modern philosophers develop clear, systematic theories of the soul? Or did they tend to separate theological questions from their own work. Are there any aspects of such immaterial "pre-psychology" that retain a lasting interest?

Answer 1

I think it is pretty clear that Descartes thought of the soul as a distinct substance, given his concept of the soul interacting with the body via the pineal gland. I also think that he identified the soul with the mind, hence his insistence on substance dualism and not property dualism/Aristotelean hylomorphism.

This hints at him considering the soul a discrete entity. This is at least implied by Berkeley's contrasting his subjective idealism ("we are all just thought blobs in the mind of God") with Descartes rationalist substance dualism.

In the same vain as Descartes, most philosophers identified soul and mind. Here's Russell on the topic in "Analysis of Mind":

We say: "I think so-and-so," and this word "I" suggests that thinking is the act of a person. Meinong's "act" is the ghost of the subject, or what once was the full-blooded soul.

And later in the same text quoting William James:

William James's view was first set forth in an essay called "Does 'consciousness' exist?"* In this essay he explains how what used to be the soul has gradually been refined down to the "transcendental ego," which, he says, "attenuates itself to a thoroughly ghostly condition, being only a name for the fact that the 'content' of experience IS KNOWN. It loses personal form and activity—these passing over to the content—and becomes a bare Bewusstheit or Bewusstsein uberhaupt, of which in its own right absolutely nothing can be said. I believe (he continues) that 'consciousness,' when once it has evaporated to this estate of pure diaphaneity, is on the point of disappearing altogether. It is the name of a nonentity, and has no right to a place among first principles. Those who still cling to it are clinging to a mere echo, the faint rumour left behind by the disappearing 'soul' upon the air of philosophy"

Answer 2

According to Descartes, the human soul is a separate substance, not a property of other substances.

Descartes conceives the soul as the human organ of thinking. i.e. as the mind (res cogitans). As such the soul is a separate substance. Each person has his own soul.

Descartes’ anthropology comprised on the other hand the substance of the body (res extensa). Hence, man comprises two substances.

This dualistic model prompts the question, how both substances interact. Descartes identified the pineal gland as the location where both interact. See

http://plato.stanford.edu/entries/pineal-gland/

The main point of Descartes’ concept of the soul was not a theological point like immortality of the soul. Instead it was the anthopological issue of the mind-body relation.

Today these questions are dealt with in the domain of philosophy of mind. Descartes’s dualistic position is discussed within the philosophy of mind sometimes also today. But only a few modern researches, e.g. Eccles and Penrose, adhere to a dualism alike to Descartes.

Modern philosophers did not invent “clear, systematic theories of the soul”. Progress has been made by researchers from psychology and from cognitive science as part of neuroscience. The most important point is to abandon the view that the soul is a substance. Instead one studies the capabilities of our nervous system. Hence one replaces the substance concept by the concept of psychic or mental functions.