Brentano takes issue with Descartes' criterion of extension being the fundamental difference between physical and mental phenomena, quoting others as having argued that "many physical phenomena appear to be without extension" and "certain mental phenomena also appear to be extended". Because of these doubts, he believes that a better way is needed to distinguish the two types of phenomena.

In fact, none of the examples Brentano supplies is convincing. Descartes says that each substance "has one principal property...to which all its other properties are referred" [PP1:53] - in the case of material substance, that property is extension, and all of the examples quoted by Brentano can be seen to be aspects of extension. For example sounds are caused by vibration, odours by very small particles of matter. Similarly, all of the examples of mental phenomena having spatial location are weak: the fact that I feel a pain in my arm does not mean the pain is located in my arm, rather that my brain has learned to associate a particular physical sensation with a particular part of my body.

Brentano suggests a better way to distinguish between mental phenomena and physical phenomena is by using the concept of intentionality: he says that "the reference to something as an object is a distinguishing characteristic of all mental phenomena. No physical phenomena exhibits anything similar".

On the second point, I think Brentano is on solid ground. Although we might say a painting is about its subject, or that a computer simulation represents a nuclear power station, the painting or computer program only exhibit a sense of "about-ness" by virtue of the fact that a person is able to notice it: the objects themselves have no conscious sense of being "about" anything.

As to whether all mental phenomena are intentional, I think the issue is less certain. There are certainly mental states which do not appear to be "about" anything in particular, for example being depressed. But just as there is an initial temptation for the depressed person to blame his feelings on something in the world, i.e. to feel that he is depressed "about" something, I think that many of the mental phenomena that appear intentional are in fact not directed to the objects which we might think.

For example, the desire to own a 36" plasma TV would appear to be a mental state where the intentional object is the TV. But there are two points to be made here: firstly, it's not a specific TV I want (I don't care what the serial number is), I want a particular type of TV. Secondly, what I desire is not the TV at all, but the pleasurable sensations that I associate with owning one (being thrilled by watching movies, showing it off to my mates, etc.). The way we express our thoughts and desires, such as saying "I want a TV", encourages the idea of mental states being intentional, but I believe that this may be misleading.