Physicalism is the thesis that in some fundamental sense, everything in the universe is physical—even things that don’t appear at first glance to be physical, such as biological, psychological, or social phenomena. Applied to human beings, it says that the capacities to think, feel, speak, and reason are in the end physical capacities determined by the makeup of brains, bodies, and physical environments. Physicalism is an ontological position: It concerns what human beings and other creatures are like. It is not a methodological or epistemological position, that is, a view about how they should be studied. It does not entail that cognitive science should be reduced to neuroscience or physics, even in the long run. It entails rather that the things studied by cognitive science are physical. Within cognitive science and philosophy, physicalism is a near-universal position. To deny it seems to involve adopting what is usually called ‘dualism,’ on which humans are complex of a physical body and an immaterial mind. It is not generally thought that such dualist alternatives to physicalism are logically impossible. It is rather that, from an empirical point of view, physicalism is true, and hence cognitive science, and science in general, should proceed on that assumption.

History

The idea of physicalism (usually going by its traditional label ‘materialism’) is very old (Wolfe & Symons, 2025). The ancient Greek thinker Democritus is often credited with a version of it when he allegedly said, “all is atoms and the void.” In the early modern period in Europe, some philosophers and scientists were materialists, but most followed Immanuel Kant (1999) in warning against what he called a “soulless materialism.”

The term physicalism (rather than materialism) became prominent in the twentieth century because of its use by philosophers such as Otto Neurath (1983) in the 1930s. Neurath is often interpreted as holding a linguistic version of the view, on which sentences describing psychological phenomena and sentences describing physical events in the brain are equivalent in meaning. In that form, physicalism is implausible; it is unlikely that a sentence describing someone’s psychological condition—for example, “Rudolph is anxious”—literally means the same as a sentence describing their physical makeup. However, this linguistic version of physicalism was transformed by later philosophers such as Quine (1960) back into an ontological thesis about the world as such and about human beings as part of the world.

Within philosophy and related areas of science, physicalism became a dominant position around the 1960s and has remained so. One major reason is the so-called causal argument for physicalism (Papineau, 2001). This argument says that the world in which human beings find themselves is a physically closed system in this sense: Everything in it has a physical cause, at least if it has a cause. It follows that human action and behavior likewise have physical causes. If one accepts, as seems plausible, that human action and behavior are caused by psychological states such as beliefs, desires, and perceptions, it follows also that these states are themselves physical, since they are physical causes of human action and behavior.

Core concepts

The core concepts associated with physicalism naturally emerge when we ask how to formulate the doctrine clearly. As we have seen, a linguistic version of the doctrine is implausible. But various ontological versions confront problems of their own.

One traditional suggestion is that physicalism entails the literal identity of psychological states with neurophysiological states. The key difficulty for this version of physicalism is that it excludes the possibility of the multiple realizability of psychological states by neurophysiological states [see Multiple Realizability] (Putnam, 1975; Fodor, 1974). The same psychological state can occur in creatures that are quite different biologically, and perhaps even in AI systems that have no biology in the normal sense. This possibility would be excluded by the identity version of physicalism.

In light of the inadequacy of linguistic or identity versions of physicalism, many theorists today formulate physicalism by saying that psychological states are ontologically dependent on physical states, even if they are not strictly identical with them. There is at present a large literature on what ontological dependence might be.

One prominent view is that ontological dependence should be understood in terms of supervenience, where, roughly, psychological properties are said to ‘supervene’ on physical properties just in case any possible situation in which the relevant physical properties are instantiated is a situation in which the relevant psychological properties are instantiated (Lewis, 1983; Kim, 1993; Jackson, 1998). However, this version of physicalism is sometimes thought to be too weak, as it apparently leaves physicalism compatible with its traditional rival, dualism (Jackson, 2006).

A different view of ontological dependence that has gained influence recently appeals to an idea known as grounding, where, roughly, psychological properties are said to be ‘grounded’ in physical properties just in case the relevant psychological properties hold by virtue of the relevant physical properties (Bennett, 2017), somewhat in the way that an apple is red in virtue of being a specific shade of red. However, whether this notion explains ontological dependence or names it is a matter of debate (Wilson, 2014; Schaffer, 2016).

Questions, controversies, and new developments

Apart from ontological dependence, there are two main sources of controversy for physicalism. The first has to do with consciousness. Many philosophers have offered general arguments that physicalism cannot be true because consciousness is not ontologically dependent on the physical (Chalmers, 1996; Jackson, 1986).

One such argument, known as the knowledge argument, says that if physicalism is true, then (at least in principle) any subject who knew all the physical facts would have a complete understanding of all the facts [see also Conceptual Analysis]. But there are compelling examples of possible subjects who know all the physical facts and yet are not able to work out all the facts about consciousness. One of these is Jackson's (1986) thought experiment about Mary, a super scientist confined to a black and white room. In principle, Mary could know everything physical there is to know about color, such as the neurophysiology of color vision, but she wouldn’t fully understand what it’s like to see blue until she comes out of the room and looks up at the sky.

The second controversy has to do with the category of the physical. What exactly is it for something to be physical? What is included and what is not? If you don’t have an answer to this question, at least in outline, it is hard to formulate physicalism or assess whether it is true or false (Hempel, 1969; Montero, 1999).

An important recent development in thinking about physicalism connects the two sources of controversy just mentioned (Montero, 2010). Suppose one defines the physical and physicalism in terms of ideal physics, the physical theory, whatever it may be, that happens to be true in our world. Is it so clear now that the knowledge argument goes through? Someone who knows all that contemporary physical science says about color vision may indeed fail to know what it’s like to see blue, but what about someone who knows all that ideal physics says? Since that is much harder to say, the issue of how to formulate physicalism has an impact on its truth.

Broader connections

Historically, philosophers have been interested in articulating and evaluating worldviews: general pictures of existence and everything in it. On the face of it, physicalism is as good a candidate for a worldview as you could imagine. It is based in science. It distinguishes ontological and methodological questions. It promises a general framework for thinking about the relation between cognitive science and other sciences. Examined closely, however, it becomes surprisingly hard to know what it is and what constraints, if any, it places on how to think about the mind. In this way, one learns again that the fact that an idea is familiar does not mean it is right.

Further reading

• Montero, B. (1999). The body problem. Noûs, 33(2), 183–200. https://doi.org/10.1111/0029-4624.00149

• Stoljar, D. (2024). Physicalism. In Edward N. Zalta & Uri Nodelman (Eds.), The Stanford encyclopedia of philosophy (Spring 2024 Edition). https://plato.stanford.edu/archives/spr2024/entries/physicalism/

• Wilson, J. M. (2014). No work for a theory of grounding. Inquiry, 57(5–6), 535–579. https://doi.org/10.1080/0020174X.2014.907542