In this passage, Descartes says that there are three types of substance, namely God, matter and mind.

God is the "primary" substance, whose existence is independent of anything else; matter and mind are substances created by, and dependent on, God. Along with Aristotle, Descartes describes attributes, or properties of substance, saying that unless substance has properties, we cannot perceive it. Additionally, although a substance may be known "through any attribute at all", all properties are aspects of a "principal" property - in the case of matter, extension, and in the case of mind, thought.

By extension, Descartes refers to the way that matter occupies physical space in the world: its position, shape, and movement. If matter didn't occupy any space then we should not be able to perceive it.

Similarly, we can only perceive the substance of mind with reference to the thoughts that it is capable of: introspection, memories, emotion etc., are all modes of thought.

Science seems to hold a similar view of the physical universe as Descartes (reflected to a certain extent in the nomenclature, e.g. "atoms", "elementary particles"): although it hasn't yet identified it, the belief appears to be that everything is made up from the same fundamental "stuff".

It is much more difficult to adopt the idea of mind - a substance that has no physical features - into the scientific view. Neuroscientists such as Vilayanur Ramachandran, who presented this year's Reith Lectures believe that all aspects of "mind" are the result of purely physical processes and chemical reactions in our brain.

There does seem to be fairly convincing evidence that Descartes' properties of mind (i.e. modes of thought) can be mapped to physical events. For example, different parts of the brain appear to be associated with distinct types of intellectual activity, and, when physically manipulated (by chemicals or some kind of trauma) can provoke a range of emotional experiences.

But it seems to me that even if there is one-to-one correspondence between physical and mental events, this does not mean that the substance of mind does not exist. Since mind has no material aspect, there is no way that science could detect it, even though it may be able to observe material consequences of the mind's operation.