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Why do we need philosophy of psychology, in addition to metaphysical philosophy of mind and psychology as distinct disciplines? Why can’t philosophers confine themselves to purely metaphysical issues? Why can’t psychologists confine themselves to purely empirical investigations?

There has been no sharp distinction between philosophy and psychology in Western intellectual history. Many philosophers who talk about mind and cognition in the history of philosophy go beyond the field of philosophy of mind in a narrow sense and touch on issues that now belong to the field of psychology, for example: Plato’s tripartite theory of soul (according to which the soul is divided into reason, spirit, and appetite) in The Republic; Descartes’ physiological analysis of various emotions in Passions of the Soul; Hume’s associationist psychology in A Treatise of Human Nature; Kant’s classification of mental disorders in Essay on the Maladies of the Head, and so on. The sharp distinction between philosophy and psychology is a modern development, mainly due to the specialization of each field. The advancement of research in each field obliged some researchers to focus on the metaphysical issues of the mind, and other researchers to dedicate themselves to empirical investigations instead. This is an inevitable and perhaps good development overall, but it invites a potential problem: many of the fundamental questions about human nature require philosophical argumentation as well as empirical inputs.

Suppose that you are interested in whether humans have some property X, such as the property of being rational or the property of being altruistic. To answer this, you need to investigate two sets of questions: (1) ‘What does it mean to have X?’, ‘What is necessary for a person to have X?’, and ‘What is sufficient for a person to have X?’; and (2) ‘Do humans satisfy a sufficient condition for having X?’, ‘Do humans fail to satisfy a necessary condition for having X?’, and ‘What do empirical studies say about these issues?’ Let us call the former the ‘philosophical questions’ about X, and the latter the ‘psychological questions’ about X.

Addressing the philosophical questions requires the articulation and justification of certain (controversial) claims, such as ‘being rational’ means reasoning in accordance with the rules of logic and mathematics, or ‘being altruistic’ means being motivated to increase somebody else’s well-being for its own sake. Addressing the psychological questions requires setting up laboratory experiments and observations in the wild, where human behaviour is assessed on the basis of the criteria for rationality or altruism, such as measuring how many people out of 100 commit a logical fallacy in a simple test or how many people out of 100 act in a selfless way given the chance.

The reason you can’t just let philosophers do what they do is that although a purely philosophical investigation is useful for addressing the philosophical questions, it does not say much about the psychological questions. For instance, a philosophical argument does not say anything about whether as a matter of fact humans can be motivated to increase somebody else’s well-being for its own sake. In contrast, the reason you can’t just let psychologists do what they do is that although a purely psychological investigation is useful for addressing the psychological questions, it does not say much about the philosophical questions. For example, a psychological experiment does not say anything about whether ‘rationality’ should be regarded as the capacity for reasoning in accordance with the rules of logic and mathematics.

Let us think about a further example (which we discuss in detail in Chapter 6). Suppose that you want to know whether humans have free will and responsibility, according to relevant psychological findings. To know this, you need to investigate both the philosophical and the psychological questions: (1) ‘What does it mean to have free will?’, ‘What is necessary for a person to have free will?’, ‘What is sufficient for a person to have free will?’; (2) ‘Do humans satisfy a sufficient condition for having free will?’, ‘Or, do humans fail to satisfy a necessary condition for having free will?’, ‘What do empirical studies say about these issues?’

A purely philosophical investigation can address the philosophical questions, but not the psychological questions. For example, one might argue, for some philosophical reasons, that it is necessary for person A to have free will that A’s conscious mental states and processes play some significant causal role in producing or controlling A’s behaviour. This is certainly a possible answer to a philosophical question, but we need to know more to ascertain whether humans have free will. In particular, the psychological questions have not been answered yet. It is still unclear whether, according to relevant empirical studies, conscious states and processes in human cognition play the right kind of causal roles. As we shall see in Chapter 6, this is the topic of a recent interdisciplinary debate on free will.

A purely psychological investigation can address the psychological questions, but not the philosophical questions. For example, psychological studies might suggest that Elsa’s discriminatory behaviour is generated by her implicit biases, which can only be detected by psychological tests. Elsa is not introspectively aware of her biases. This is certainly an interesting finding, but we need to know more to judge whether she is responsible for her discriminatory behaviour. In particular, the philosophical questions have not yet been addressed. It is still unclear whether Elsa’s lack of introspective awareness of her implicit biases is incompatible with her being responsible for her discriminatory behaviour. As we shall see in Chapter 6, this is the topic of a recent discussion about whether people are responsible for their implicit biases.