the branch of philosophy that includes the philosophy of psychology, philosophical psychology, and the area of metaphysics concerned with the nature of mental phenomena and how they fit into the causal structure of reality. Philosophy of psychology, a branch of the philosophy of science, examines what psychology says about the nature of psychological phenomena; examines aspects of psychological theorizing such as the models used, explanations offered, and laws invoked; and examines how psychology fits with the social sciences and natural sciences. Philosophical psychology investigates folk psychology, a body of commonsensical, protoscientific views about mental phenomena. Such investigations attempt to articulate and refine views found in folk psychology about conceptualization, memory, perception, sensation, consciousness, belief, desire, intention, reasoning, action, and so on. The mind–body problem, a central metaphysical one in the philosophy of mind, is the problem of whether mental phenomena are physical and, if not, how they are related to physical phenomena. Other metaphysical problems in the philosophy of mind include the free will problem, the problem of personal identity, and the problem of how, if at all, irrational phenomena such as akrasia and self-deception are possible. Mind–body dualism Cartesian dualism. The doctrine that the soul is distinct from the body is found in Plato and discussed throughout the history of philosophy, but Descartes is considered the father of the modern mind–body problem. He maintained that the essence of the physical is extension in space. Minds are unextended substances and thus are distinct from any physical substances. The essence of a mental substance is to think. This twofold view is called Cartesian dualism. Descartes was well aware of an intimate relationship between mind and the brain. (There is no a priori reason to think that the mind is intimately related to the brain; Aristotle, e.g., did not associate them.) Descartes (mistakenly) thought the seat of the relationship was in the pineal gland. He maintained, however, that our minds are not our brains, lack spatial location, and can continue to exist after the death and destruction of our bodies. Cartesian dualism invites the question: What connects the mind and brain? Causation is Descartes’s answer: states of our minds causally interact with states of our brains. When bodily sensations such as aches, pains, itches, and tickles cause us to moan, wince, scratch, or laugh, they do so by causing brain states (events, processes), which in turn cause bodily movements. In deliberate action, we act on our desires, motives, and intentions to carry out our purposes; and acting on these mental states involves their causing brain states, which in turn cause our bodies to move, thereby causally influencing the physical world. The physical world, in turn, influences our minds through its influence on our brains. Perception of the physical world with five senses – sight, hearing, smell, taste, and touch – involves causal transactions from the physical to the mental: what we perceive (i.e., see, hear, etc.) causes a sense experience (i.e., a visual experience, aural experience, etc.). Thus, Descartes held that there is two-way psychophysical causal interaction: from the mental to the physical (as in action) and from the physical to the mental (as in perception). The conjunction of Cartesian dualism and the doctrine of two-way psychophysical causal interaction is called Cartesian interactionism.
Perhaps the most widely discussed difficulty for this view is how states of a non-spatial substance (a mind) can causally interact with states of a substance that is in space (a brain). Such interactions have seemed utterly mysterious to many philosophers. Mystery would remain even if an unextended mind is locatable at a point in space (say, the center of the pineal gland). For Cartesian interactionism would still have to maintain that causal transactions between mental states and brain states are fundamental, i.e., unmediated by any underlying mechanism. Brain states causally interact with mental states, but there is no answer to the question of how they do so. The interactions are brute facts. Many philosophers, including many of Descartes’s contemporaries, have found that difficult to accept.
Parallelism. Malebranche and Leibniz, among others, rejected the possibility of psychophysical causal interaction. They espoused versions of parallelism: the view that the mental and physical realms run in parallel, in that types of mental phenomena co-occur with certain types of physical phenomena, but these co-occurrences never involve causal interactions. On all extant versions, the parallels hold because of God’s creation. Leibniz’s parallelism is preestablished harmony: the explanation of why mental types and certain physical types co-occur is that in the possible world God actualized (i.e., this world) they co-occur. In discussing the relation between the mental and physical realms, Leibniz used the analogy of two synchronized but unconnected clocks. The analogy is, however, somewhat misleading; suggesting causal mechanisms internal to each clock and intramental and intraphysical (causal) transactions. But Leibniz’s monadology doctrine excludes the possibility of such transactions: mental and physical phenomena have no effects even within their own realms. Malebranche is associated with occasionalism, according to which only God, through his continuous activities, causes things to happen: non-divine phenomena never cause anything. Occasionalism differs from preestablished harmony in holding that God is continually engaged in acts of creation; each moment creating the world anew, in such a way that the correlations hold.
Both brands of parallelism face formidable difficulties. First, both rest on highly contentious, obscure theological hypotheses. The contention that God exists and the creation stories in question require extensive defense and explanation. God’s relationship to the world can seem at least as mysterious as the relationship Descartes posits between minds and brains. Second, since parallelism denies the possibility of psychophysical interaction, its proponents must offer alternatives to the causal theory of perception and the causal theory of action or else deny that we can perceive and that we can act intentionally. Third, since parallelism rejects intramental causation, it must either deny that reasoning is possible or explain how it is possible without causal connections between thoughts. Fourth, since parallelism rejects physical transactions, it is hard to see how it can allow, e.g., that one physical thing ever moves another; for that would require causing a change in location. Perhaps none of these weighty difficulties is ultimately insuperable; in any case, parallelism has been abandoned. Epiphenomenalism. Empirical research gives every indication that the occurrence of any brain state can, in principle, be causally explained by appeal solely to other physical states. To accommodate this, some philosophers espoused epiphenomenalism, the doctrine that physical states cause mental states, but mental states do not cause anything. (This thesis was discussed under the name ‘conscious automatism’ by Huxley and Hogeson in the late nineteenth century. William James was the first to use the term ‘epiphenomena’ to mean phenomena that lack causal efficacy. And James Ward coined the term ‘epiphenomenalism’ in 1903.) Epiphenomenalism implies that there is only one-way psychophysical action – from the physical to the mental. Since epiphenomenalism allows such causal action, it can embrace the causal theory of perception. However, when combined with Cartesian dualism, epiphenomenalism, like Cartesian interactionism, implies the problematic thesis that states of an extended substance can affect states of an unextended substance. An epiphenomenalist can avoid this problem by rejecting the view that the mind is an unextended substance while maintaining that mental states and events are nonetheless distinct from physical states and events. Still, formidable problems would remain. It is hard to see how epiphenomenalism can allow that we are ever intentional agents. For intentional agency requires acting on reasons, which, according to the causal theory of action, requires a causal connection between reasons and actions. Since epiphenomenalism denies that such causal connections are possible, it must either maintain that our sense of agency is illusory or offer an alternative to the causal theory of action. Similarly, it must explain how thinking is possible given that there are no causal connections between thoughts. Monism
The dual-aspect theory. Many philosophers reject Descartes’s bifurcation of reality into mental and physical substances. Spinoza held a dualattribute theory – also called the dual-aspect theory – according to which the mental and the physical are distinct modes of a single substance, God. The mental and the physical are only two of infinitely many modes of this one substance. Many philosophers opted for a thoroughgoing monism, according to which all of reality is really of one kind. Materialism, idealism, and neutral monism are three brands of monism. Hobbes, a contemporary of Descartes, espoused materialism, the brand of monism according to which everything is material or physical. Berkeley is associated with idealism, the brand of monism according to which everything is mental. He held that both mental and physical phenomena are perceptions in the mind of God. For Hegel’s idealism, everything is part of the World Spirit. The early twentieth-century British philosophers Bradley and McTaggart also held a version of idealism. Neutral monism is the doctrine that all of reality is ultimately of one kind, which is neither mental nor physical. Hume was a neutral monist, maintaining that mental and physical substances are really just bundles of the neutral entities. Versions of neutral monism were later held by Mach and, for a short time, Russell. Russell called his neutral entities sensibilia and claimed that minds and physical objects are logical constructions out of them.
Phenomenalism. This view, espoused in the twentieth century by, among others, Ayer, argues that all empirical statements are synonymous with statements solely about phenomenal appearances. While the doctrine is about statements, phenomenalism is either a neutral monism or an idealism, depending on whether phenomenal appearances are claimed to be neither mental nor physical or, instead, mental. The required translations of physical