Descartes' view of the world is that created substance is of two types: extended substance ("body", or "matter" to use Ryle's term) and thinking substance ("mind"). Although they are fundamentally different, they are both types of substance, and are treated in cartesian metaphysics as being of equivalent status. Mind and body are distinct from one another, and just as it is possible for a mindless body to exist (for example, a wheelbarrow), Descartes says that a mind can (logically) exist without requiring a body to contain it.

Ryle says that the Cartesian view is based on a "category-mistake": while he grants that the concepts of "mind" and "body" are useful and valid ones, and that it is logical to say that each exists, "these expressions do not indicate two different species of existence...[but] two different senses of 'exist'".

In the extract, Ryle describes and gives some examples of category-mistakes. In each example, one thing is mistakenly treated as being in the same category as another thing or set of things. So we have "university", "division", "pair", and "flood of tears", which are collective terms or metaphors, being treated as if they were concrete physical entities. On the basis of these examples, it seems reasonable to accept that such a thing as a "category-mistake" exists and that we are capable of making them. The issue is whether this is what Cartesian dualism is guilty of.

Ryle criticises Descartes for being unwilling to accept that "human nature differs only in degrees of complexity from clockwork", an idea that Ryle implicitly endorses. According to this view, "mind" is no more than a useful metaphor to describe the collection of physical processes that occur as a consequence of possessing a highly evolved brain. And on this basis, we would be forced to conclude that to contrast "mind" and "matter" would be to commit a category-mistake.

However, I think Ryle's argument is based on a false premise (that mind is just a way to describe the physical functions of the brain) and therefore open to challenge. He says that in talking of a person's mind, we are referring to "abilities, liabilities and inclinations" (where an inclination is no more than an impression gained by past experience of what the person would do in a particular situation), i.e. externally observable events.

It seems though that there are aspects of mind which do not have corresponding external features and so are not explicable in this way. For example, if I am sitting in front of a keyboard, looking into the distance, it would not be not possible for an observer, even one equipped with brain scanning equipment, to capture the subjective experience I am having; it is not something which can be described in purely mechanical (i.e. non-mental) terms.