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The foregoing demonstrates the sheer diversity of what happens in the mind. Pains are as different to beliefs as beliefs are to perceptions. We bundle these diverse phenomena under the heading of ‘mental’, but what is it that makes a state mental? To say that Mindy has a certain ‘state’ is just to say that Mindy has some property at a particular time. And there are countless non-mental properties that Mindy has, such as having a body temperature of 37.1° Celsius. But why don’t they qualify as mental?

Consider the table on the following page. This looks like an intuitive way of organizing Mindy’s states. Although states like Mindy’s muscle fatigue can cause mental states like the feeling of achy muscles, it remains clear that muscle fatigue itself is non-mental. So what determines whether a state goes in the first column or the second? We can ask the same question about the processes Mindy is undergoing. A process is a sequence of states that unfolds over time. For instance, Mindy is in the process of reasoning about where to aim her shot and in the process of digesting her lunch. But what makes the former process mental and the latter nonmental? To answer this, we need to find some defining feature of mentality – a feature possessed by everything in the mental column but nothing in the non-mental column. This elusive feature is known as the mark of the mental.

Table 1.1 Mental and Non-Mental Properties

MENTAL	NON-MENTAL
Perceiving the football	Having a temperature of 37.1° Celsius
Feeling an ache in her muscles	Having a heart rate of 125 beats per minute
Feeling excited	Having a blood pressure of 100/70
Believing that the goalkeeper will go left	Having muscle fatigue
Desiring that she will score	Being well hydrated
Remembering that the goalkeeper went left before	Being 6ft tall
Having an intention to kick the ball	Being in good physical health

A tempting proposal is that mental states are distinguished by being states of the mind. Notice that everything in the first column is a state of Mindy’s mind while everything in the second column is a state of Mindy’s body. The problem with this proposal is that it just relocates the question we were trying to answer. Now we face the question of what makes something a state of the mind rather than a state of the body and we’re no better off than we were. Another possible response is that there is no mark of the mental. We apply the label ‘mental’ to some states and not others, but our groupings are more or less arbitrary. On this view, there’s no interesting feature that marks out all the states in the first column. One difficulty with this proposal is that grouping mental states together seems far from arbitrary. Intuitively, there is something these states have in common, even if that ‘something’ is hard to pin down. The sceptical response would also be bad news for our theorizing about the mind. We want to understand what mental states are, how we know about them and which things possess them. Anyone claiming that the category ‘mental’ is spurious will have to deny that these are worthwhile questions to investigate because they employ arbitrary categories.

A more promising proposal is that the mark of the mental is intentionality. The word ‘intentionality’ sounds like it should have something to do with a person’s intentions, but this appearance is misleading. The word is derived from medieval Latin, and to have intentionality is to be about something. Mindy’s perceptual experience, for example, is a perception of the football. So although her perceptual state is something in her mind, that state is about something beyond itself, namely the football. We can call the target of an intentional state an intentional object. Going through Mindy’s other mental states, it’s not too hard to pick out their intentional objects. Her achy feeling is about her muscles, her excitement is about her prospective goal, her desire is about scoring and her belief and memory are about the goalkeeper. In contrast, Mindy’s non-mental states don’t seem to be about anything. Mindy’s height and muscle fatigue aren’t about anything – they just exist without pointing beyond themselves.

An interesting feature of intentionality is that something can have an intentional object even when that object does not exist. A desire to find the Holy Grail is about the Holy Grail, even if no such object exists. A perceptual experience of a floating dagger is about a dagger, even though no such dagger is present. A belief in fairies is about fairies, even though there are no such creatures. We can make sense of this distinctive feature of intentionality by making an analogy with paintings. Some paintings are paintings of real things. Holbein’s portrait of Henry VIII, for example, is of a real flesh-and-blood person. Other paintings are not of real things. Burne-Jones’s painting The Beguiling of Merlin is a painting of Merlin, even though no such magician exists. So the fact that a painting is about something does not entail that thing exists. Similarly, a mental state being about something does not entail that thing is real. Mental states and paintings both point beyond themselves to something else, and they can do so regardless of whether there’s anything real they are pointing to (this feature of intentionality is a philosophical rabbit hole down which we won’t be going, but some readings that do venture down the hole can be found at the end of the chapter).

So far we’ve seen that at least some mental states have the property of intentionality and some non-mental states lack intentionality. But for intentionality to be the mark of the mental, something much stronger is needed: it must be the case that all and only mental phenomena have intentionality. The thesis that intentionality is necessary and sufficient for mentality was named Brentano’s thesis after Franz Brentano (1838–1917). Although this idea has a long history running back at least to Aristotle, Brentano made a particularly influential case for it. One motivation for Brentano’s thesis is the thought that to have a mind is to have a perspective on the world – a point of view. Mindy has one perspective on the world, the goalkeeper has a different perspective, and the referee another. The mindless football, on the other hand, has no perspective at all. Nor do the goalposts or the referee’s whistle. Having a perspective means having a perspective on or about something. So perspectives come hand-in-hand with intentionality. Mindy’s perspective on the world is made up of all her intentional states: her beliefs about things in the world, her perceptions of them, her feelings about them, and so on. The goalkeeper’s perspective is made up of a completely different set of mental states: different beliefs, perceptions, feelings and so on. What makes those states mental states is that they are constituents of a perspective, and what makes them constituents of a perspective is their intentionality. Mindy’s non-intentional states cannot be constituents of her perspective on the world, and this is what makes them non-mental. If you buy into this equation of minds with perspectives, you’re a long way towards agreeing with Brentano’s thesis.

To properly evaluate Brentano’s thesis we need to consider whether it is vulnerable to counter-examples. Can we undermine Brentano’s thesis with a non-mental case of intentionality? Well, we’ve seen already that paintings can have intentionality, like a portrait being of Henry VIII. Similarly, the map in my drawer is about Cambridge, the book on my desk is about philosophy and the reading on my thermostat is about the temperature. All of these things have intentional objects, yet none of them plausibly have mental states. Advocates of Brentano’s thesis deal with such cases by arguing that these things only have intentionality because we give it to them. The painting is a painting of Henry VIII (rather than of his brother or of a fictional king) because that’s who Holbein meant it to be of. The map is of Cambridge because the map-makers designed it to be. And the reading on the thermostat is about the temperature because that is the function it was given. A bunch of stuff happening in a box on the wall is meaningless without the wider context of people who design and use thermostats. On this view, one of the things that beings with real intentionality can do is imbue non-mental things with this kind of derivative intentionality. But non-derivative intentionality remains an exclusively mental property.

Can we undermine Brentano’s thesis with a mental state that is non-intentional? It’s unlikely that perception will provide such a counter-example. We normally describe our perceptual states in terms of what they’re about – a perception of a cat, or of a cup or of a cake. In fact, it is hard to see how something could be a perception without being a perception of something. It’s also unlikely that propositional attitudes will provide a counter-example. A propositional attitude is about whatever figures in the proposition. It also looks like intentions are inevitably intentional (though not because of the superficial similarity of the terms). Intentions are directed at whatever they are intentions to do. Pains might cause more trouble. One might argue that pains aren’t really about anything – they just are. Emotions can also cause difficulties. Although emotions like Mindy’s elation have a clear intentional object, other emotions seem to be undirected. Your mood might be cheerful, or grumpy, or melancholy, yet none of these emotions need to be about anything in particular. Maybe we can say that these emotions are about the world in general but there would have to be good arguments for understanding them that way.

The claim that intentionality is the mark of the mental certainly deserves to be taken seriously. But even if we stop short of advocating Brentano’s thesis, the foregoing provides us with something useful. First, it gives us some idea of how to draw the line between the mental and the non-mental, and thus of how to delineate the subject matter of philosophy of mind. Second, it gives us the valuable concept of intentionality to put in our conceptual toolkit. My initial sketch of what was going on in Mindy’s mind was a sketch of her perspective – her take on the world – with different mental states contributing to that perspective in their own distinctive ways. Understanding the mind will at least partly be a matter of understanding someone’s perspective, and we can apply that insight as we begin to explore the big questions that define the philosophy of mind.