[Image] Confucius Tsze-lu said, "The ruler of Wei has been waiting for you, in order with you to administer the government. What will you consider the first thing to be done?"

The Master replied, "What is necessary is to rectify names."

The Analects. (13.1)
The three words in the title of this essay are often conflated and used pejoratively to criticise anyone who argues that the results of scientific exploration must be taken into account. In fact they delineate three different philosophical narratives, the first two are ontologies concerned with the nature of reality, while the latter is an epistemological position. Since the terms come up so often and are so often used indiscriminately, leading to confusion, it's worth unpacking them and sorting one from the other.

Physicalism.

Physicalism is a relatively new word. It was coined in the 1930's by the Vienna Circle, a group of philosophers, mathematicians and scientists, which is also associated with the epistemological stance of Positivism. Indeed the confusion of anti-science campaigners is such that they will often refer to science as "Positivist".  This is very easy to refute since in the schism between Viennese refugee Karl Popper and the Vienna Circle, scientists decisively sided with Popper in rejecting Positivism. Modern science is not Positivist, it is, if anything, Popperian. The heart of the dispute was the Positivist claim that propositions could only be considered true when they could be directly verified. Popper showed, using the example of the black swan, that this was not a useful approach to assessing the truth value of knowledge. For example: "All swans are white" had been used as an example of an demonstrably true proposition in Europe, since all European swans are white. But in Australia swans are black and thus once Europeans got to Australia they realised that it was never true that all swans were white. This is now known as the Black Swan Effect. The Positivist approach is constantly undermined by unknown unknowns. Those who claim there is no certain knowledge cite the Black Swan Effect as a justification for this view. 
The Physicalist position is essentially a linguistic one. They said that all linguistic statements are synonymous with some physical statement. Which boils down to the idea that everything is (ultimately) physical. If this were so it would certainly make truth claims a lot easier to establish or test. Everything we experience is simply a result of how the physical world is arranged. For example an arrangement of atoms.
Although philosophers still discuss the idea of Physicalism, it is not a very convincing position and has very little influence on the world at present. Indeed it is precisely the mind which undermines physicalism. It is very difficult to account for the phenomena of the mind in a Physicalist paradigm. While most current theories of mind are reductive, in the sense of explaining the mind as an activity of the brain, this would still be difficult to account for on the basis of Physicalism, because the phenomena of the mind are not physical. For some philosophers this looks like a case for substance dualism. David Chalmers who coined the term "The Hard Problem" is a substance dualist. 
I think it's safe to say that no scientist is presently trying to explain the mind through the Physicalist paradigm. Granted, the physicists seeks to understand physical phenomena through studying the physical world. But this is a methodological approach rather than an ontological position. Physicists may believe that studying the world (the way they do) will lead to a theoretical understanding of reality, but this is technically not Physicalism, it is Naturalism. 

Materialism 

Materialism is a somewhat older term with roots in the early Enlightenment. We need to think carefully about the historical context of Materialism. In fact some of the Ancient Greeks were materialists - they believed that the world was made up of one substance and it's transformations. A popular early contender for this single substance was water. Fire was also considered by some. There were apparently some materialists in ancient India as well and they also played around with both water and fire as the ultimate substance. A little note here is that in Buddhism we frequently meet Nihilists who do not believe in rebirth, or Determinists who believe our actions are all pre-determined, but neither of them can legitimately (or rationally) be called Materialists because they do not espouse a substance ontology. However it is de rigueur to irrationally call such characters materialists. Materialism, as an ontology, did not catch on either in Europe or in India. In Europe materialism lost ground to other ideas and then was obliterated by Christianity for over 1000 years. In India the transmigration of souls in a cyclic eschatology required some form of Vitalism that dominates the Indian worldview even today.
At the dawn of the Enlightenment the Roman Catholic Church (previously the Holy Roman Empire) had been the dominant intellectual power for a millennia. They maintained this by having a monopoly on education and by persecuting heretics. Roman theology translated into secular power as well. Thus when the first cracks appeared in Church dogma - discoveries by Johannes Kepler, Copernicus, Tycho Brahe and Galileo - they were embraced with great enthusiasm in some quarters where the Church was less popular.

As much as anything the early Materialists hoped to throw off the oppressive yoke of the Church. And they did this by playing up the possibilities of gaining knowledge by studying the material world as distinct from the spiritual world of the Church; and by playing down the superstition and ignorance fostered by the Church as part of its program to control the masses.

We have to see the Materialism of the Enlightenment as distinct from contemporary Materialism because of the historical context and the fact that most of the central planks of contemporary materialism were discovered in the 20th century. The understanding of the 17th and 18th Century materialists was entirely different and commentators such as Arnold Schopenhauer (the darling of many Romantics) who attempt to refute 18th century Materialism are barely relevant to modern discussions of materialism because they are talking about something completely different.

In the 21st century the Church is a spent force intellectually. For a start it is divided and full of internal strife over issues of equality. The Church plays no major role in public discourse any more. In addition we have a series of discoveries that have established materialism as a very useful way of seeing the world: building on the life and works of Newton, Hume and Kant et al.; 19th century natural philosophers extended our knowledge of the natural world: evolution and the discovery of fossils; the explorations of the early chemists; Maxwell's electromagnetism and so on. This laid the foundations for far more sophisticated theories which have allowed exploration of the natural world in greater breadth and depth, such as: Relativity, quantum mechanics, and nuclear forces using techniques such as deep space telescopes, electron microscopes, and fMRI scanners taking pictures of the brain in action. These are all the activities of what used to be called natural philosophers - those whose study is of the natural world, and who nowadays take an approach that might be called Naturalism.
The success of methodological Naturalism can lead to the ontological view that the material world is all that there is, i.e. to mono-substance materialism. However the Materialism of today is vastly different to the Materialism of the 19th Century. The picture of nature is much more wide ranging and compelling. It will readily be admitted that we do not understand everything, the mysteries of dark matter and dark energy, not to mention problems associated with the mind, are as yet unsolved. Still, what we do know about the world is astounding. And we know the basic principles upon which the world, as we know it, operates. (See Sean Carroll's essay Seriously, The Laws Underlying The Physics of Everyday Life Really Are Completely Understood).
The main problem that undermines Materialism as a complete ontology is what David Chalmers has called "the hard problem of consciousness. As he says "The really hard problem of consciousness is the problem of experience." It's very difficult to explain first person experience, the fact that we are subjects of experience, from a Materialist perspective. However we must carefully note that, rather disappointingly, Chalmers is a substance dualist: in expounding his views he makes it clear that he believes that the mind is made of a different stuff to matter. It is natural, even axiomatic, for a mind/body substance dualist to argue that studying matter will tell us nothing about the mind. Substance dualism is a theological position rather than a philosophical position: there is no way to test the proposition, it must simply be taken on faith. Just because a substance dualist like Chalmers cannot conceive of a way around the problem, because his definition of mind erects insurmountable barriers around it, does not mean that people who reject substance dualism are bound by the same assumptions. I recently cited John Searle and his contention that these discussions often mix up ontology and epistemology: "The ontological subjectivity of the domain [of consciousness] does not prevent us from having an epistemologically objective science of that domain". - (Consciousness as a Problem in Philosophy and Neurobiology) Over the last few weeks I've been arguing that substance dualism, and in particular Vitalism, is incompatible with basic Buddhism. In fact like Nāgārjuna I'm forced to conclude that any hard and fast ontological position is untenable, because by the Buddhist understanding of the existential situation there is no epistemological support for any ontology. We simply have no way to know one way or the other if the world really exists or really doesn't. All we can know is that experience arises and passes away and it marked by impermanence, disappointment and insubstantiality. However I offer the caveat that together we can infer a lot about the world and that through empiricism and comparing notes we have a lot of useful information and accurate theories. 

The other kind of Materialism, the other side of the mono-substance ontology, argues that there is only one kind of substance in the universe and it is mind. Whereas the Materialism that is regularly attacked by Buddhists is a form of Realism, if we say that there is only mind then we have a form of Idealism (after Plato's conception of 'ideas' the ultimate, true, noumena behind phenomena). Idealism is quite a popular philosophical stance amongst Buddhists.  And it's still a mono-substance ontology and thus a form of Materialism.

Scientism 

Scientism is distinct from Physicalism and Materialism because it's primarily an epistemological stance. Scientism, on the back of the massive success of science, argues that the scientific method (empiricism) is the only valid method of acquiring knowledge. Presumably Scientism would argue that common sense is a less sophisticated form of empiricism. In fact this is mostly a pejorative term used by social "scientists" against real scientists. And the irony here is that the humanities have been vigorously gearing up to be sciences since around the time "Scientism" was coined as a pejorative. So in some sense the argument is not with scientists, but with humanities scholars enthusiastically adopting the paradigms of science. Of course they do this because of the kudos that comes with empirical research: it's much harder to argue with measurement than with surmise or reflection. 
In fact I see this adoption of empiricism outside the natural sciences as a rather baleful influence on everyday life. Ordinary professionals such as teachers and nurses now have to have masters degrees and spend half their time on administrative and bureaucratic tasks designed to measure their performance. Such initiatives stem from the influence of Neolibertatian ideology on society: Neolibertarians enthusiastically adopted Game Theory for example and measurements of productivity adapted from manufacturing. The most egregious example of this was measuring the efficiency of the Vietnam War in terms of the "body count". The inventor of this metric, Allan Enhoven, was subsequently employed in the 1980s by British Prime Minister, Margaret Thatcher to reorganise the National Health Service. But the upshot of this is that there is never enough efficiency and constant organisation reviews and reorganisation that do more to sap efficiency than regular bouts of Norovirus. If there's a downside to empiricism this is it. 
The critics of science particularly focus on the reductive nature of scientific theories - things are always explained in terms of simpler components. (Which is just what Buddhism does in models like the skandhas, dhātus and nidānas). In fact though science does largely rely on reductive accounts, with huge success it must be said, this is changing with the rise of cross-discipline work and systems theory. Reductive explanations give you a particular kind of leverage on the problems you are looking at. Buddhists exploit this leverage as much as scientists to, though to different ends. 
In many ways the term Scientism expresses the anxiety that the efficacy of previously privileged forms of knowledge seeking (such as through meditation or abstract philosophy) are denied by scientists. This anxiety being felt as much within the disciplines of sociology and psychology as without. The application of empiricism to fields like psychology looks like reducing the role of gifted pioneers like Sigmund Freud. The impressionistic and visionary approach to psychology doesn't always tally with what scientists find. take homeopathy which is so popular amongst those who lean towards Buddhism. Factually speaking there is nothing in homeopathic remedies and homeopathy is exposed as based on untrue propositions. The Saturday Morning Breakfast Cereal cartoon website sells tee-shirts with the legend: Science: Ruining Everything Since 1543. The term "Scientism" is aimed at taunting those killjoy scientists who disprove unicorns and homeopathy, often with no real acknowledgement of the successes of science and the new stuff that we enjoy: like the internet.

Jan 2016: See also Sean Carroll's plea: Lets Stop Using the Word Scientism.

Ontology and Epistemology

Early Buddhism has a reasonably clear epistemology i.e. it is reasonably clear on what constitutes sources of valid knowledge (pramāṇa). Historically this clarity is lost because Buddhists begin to prioritise ontology, but before they go down that dusty road, there is some clarity. Knowledge comes from experience.
The central truth criteria are three axioms: experience is impermanent; experience is unsatisfactory; and experience is insubstantial. Any knowledge which conforms to these three axioms is valid knowledge. But here we must be cognizant of the scope of early Buddhist thought. Time and again the Buddha says: "I teach suffering, the origin of suffering, the cessation of suffering, and the way to cessation." Thus early Buddhist ideas were never intended as a philosophical system as though Gotama were an Indian Plato or Aristotle. Buddhism is programmatic. It's pragmatically focussed on duḥkha and minimalist in being unconcerned with wider philosophical questions which when asked are frequently left aside as unexplained (avyākata).
By cultivating certain kinds of experience, particularly samādhi or integration and by reflecting on experience per se in that state, one can get access to knowledge of the nature of experience (yathābhūta-jñānadarśana) and become liberated from duḥkha. Subsequent to that liberation (vimukti) we obtain the knowledge that we are liberated (vimuktijñāna) or the knowledge that we have destroyed the āsravas (āsravakṣayajñāna). 
There is another kind of knowledge traditionally associated with samādhi called abhijñā. This is what we would call "extra-sensory perception". How we understand these ESPs will depend on temperament and worldview. However the most import of the abhijñās, and the only one described as lokuttara, is āsravakṣaya the destruction of the fluxes; synonymous with vimukti. And although , in early Buddhist texts, it is certainly possible to gain ESP powers, it is not usually seen as desirable, especially in contrast to the āsravakṣaya.
At no point in early Buddhist texts, and as far as I know in the Perfection of Wisdom texts or the Sukhāvativyūha texts, does the Buddha say anything at all about the nature of reality or of objects. Such speculations as we have in the Buddhist tradition seem to come out of arguments between the successors of the Ābhidharmikas and non-Buddhist Indian philosophers and to date mainly from ca. the 6th century AD onwards. Buddhists went for over 1000 years without worrying about what the world is made of. Even the so-called "elements" (dhātu) are defined in experiential terms: earth is characterised by the experience of resistance and so on. 
It's important to be clear about all this, about the doctrinal stance that underpins Buddhism: both early Buddhism, sectarian Buddhism and at least the early Mahāyāna. The focus is on gaining knowledge that can release us from suffering. That knowledge is obtained by examining our mind, especially from a state of samādhi or through reflections carried out immediately post-samādhi. While natural processes do offer metaphors for the mind, the natural world is never given any consideration in the process of liberation. It is broadly speaking a source domain of objects of the senses, but nothing more and of little or no interest to Buddhist thinkers.
I've already mentioned that one of the implications of this Buddhist epistemology is that it can support no ontological arguments. And indeed where Buddhists make ontological arguments they have to first modify the Buddhist epistemology in ways that are not related to the program of gaining liberation from suffering. Thus, I would argue, that if one is a Buddhist then one cannot legitimately take an ontological stand. I believe that this is precisely the message of the Kaccānagotta Sutta. 
From the early Buddhist point of view, we have no basis for arguing that "reality" or "things" or "the universe" is one way or the other. We have no basis for a Realist point of view and no basis for an Idealist point of view. We have no valid source of knowledge about the nature of reality or the nature of objects of the senses. All we have is experience. And even those people with insight are just describing another kind of experience which is entirely personal to them. Knowledge from the senses can be reliable to varying degrees, even the unawakened can function in the world and physics makes incredibly accurate predictions. But any ultimate knowledge we might gain can only be of the workings of the mind, and in particular the way the mind responds to sensory stimulation and how that relates to the three axioms of experience.

Conclusions

Whenever we see pejoratives flying around in a intellectual discussion we know that someone's toes have been stepped on. Pejoratives are about trying to score points. Good polemic deals with substantive points, it does not resort to lazy labelling. Of course it can be helpful to point out that a critic has an unstated, and possibly unexamined, assumption or philosophical stance. Buddhists all too often take a stand in Romantic ideology or in Vitalist ontology. Or they may cite some anachronistic philosophy or philosopher (Schopenhauer is a favourite). And it can be helpful to point out and critique the stance or the view when developing an argument. When one's critics are thoughtlessly expounding a philosophical stance, then undermining that stance is a valid way of proceeding. Pejoratives are employed to shut down discussions, to silence opposition, and to try to put an opponent at a disadvantage so as simply to win an argument.
It's useful to see that Physicalism, Materialism, and Scientism are three different labels for three different approaches to being and/or knowledge. And to know that if one wants to put a non-polemical label on the worldview of most scientists it would be Naturalism.
If someone wants to pick a fight on the basis of their own confusion about these terms, or based on an anachronistic view of science, or the views of a philosopher who died before science really got going; or to make an argument based on an ontology for which there is no supporting epistemology; then I'm under no more obligation to take up that fight than I would be to argue theology with a Jehovah's Witness appearing uninvited on my doorstep. Wrong views are irrelevant to my project/object. Refuting the wrong views of individual strangers seldom attracts me, unless the stranger seems to have a representative view or class of views, or if in refuting a wrong view I can highlight a right view.

~~oOo~~