While probably no one today would hold to this original Cartesian form, it is worth examining because it reveals many misunderstandings that beset other dualistic approaches. First, there is no reason to suppose that, since Soul is non-spatial, it is without parts. The analogy of a computer programme shows that it may be appropriate to analyse a system into functional parts without these parts being spatially located. Second, it ignores the possibility of causal influences that act on a physical material system in a distributed way, such as occurs with superconductivity, in which the charge carriers are distributed over a large region and the mechanisms governing their behaviour are global in nature.
The overiding objection, however -- which undermines most dualistic theories -- is that in fact it fails to offer any explanation. In an attempt to explain what is happening when we see a tree it describes how a neural image of the tree is conveyed to the visual cortex (to modernise Descartes a little), but then calls in a separate soul to observe the neural image. To the question, how does the soul observe the image, no answer can be given, because the soul is assumed totally simple and so incapable of being further analysed. The theory is thus not an explanation, but a means of avoiding explanation.