Trenton Merricks is a relatively young professor of philosophy and metaphysics at the University of Virginia. He is one of the staunch defenders of mereological nihilism, the idea that there are no composite objects, only "simples" arranged to look like objects. There are "no tables, only simples arranged tablewise," said Peter van Inwagen in his 1990 book Material Beings.

Van Inwagen made an exception for living things, an abstruse argument based on Descartes' idea that humans are thinking beings and "I think, therefore I am (existing?)."

Merricks follows van Inwagen in accepting human organisms as existing objects. But he goes beyond van Inwagen by denying reductionist arguments that the physical world is "causally closed" from the "bottom up."

Merricks adapts the reductionist claims of Jaegwon Kim that say properties in a complex system can be "reduced" to the lower-level properties of the system's components. For example, the laws and properties of chemistry can be reduced to the laws of physics.

More specifically, the properties of molecules can be reduced to those of atoms, the properties of biological cells can be reduced to those of molecules, plants and animals can be reduced to those of cells, and mind can be reduced to neurons in the brain. So far, Merricks agrees, any composite object is reducible to its simples - atoms or whatever the latest physics tells us are the most fundamental material objects.

Merricks' ontology is Kim's ontology.

The most fundamental tenet of physicalism concerns the ontology of the world. It claims that the content of the world is wholly exhausted by matter. Material things are all the things that there are; there is nothing inside the spacetime world that isn't material, and of course there is nothing outside it either. The spacetime world is the whole world, and material things, bits of matter and complex structures made up of bits of matter, are its only inhabitants.

(Physicalism, Or Something Near Enough. Princeton University Press (2005), p.150)

Kim objected to attempts by other philosophers of mind to defend mental causation with a "non-reductive physicalism" and the idea of "top-down" supervening mental events, especially those proposed by Donald Davidson.

Kim is what is known as an "eliminative materialist," a position famously espoused by Daniel Dennett and the Churchlands. The positions of van Inwagen and Merricks are similarly "eliminativist."

Kim argues that mental events are redundant because for every event in a "mind," there must be a corresponding physical event in the brain that is doing the real causal work. Kim calls for "excluding" the mental events, describing them as "overdetermining" actions.

Merricks develops a powerful analogy between Kim's mental events and van Inwagen's non-existing composite objects. His prime example is a baseball breaking a window.

CONSIDER the following argument about an alleged baseball causing atoms arranged windowwise to scatter, or, for ease of exposition, causing 'the shattering of a window'.

(i) The baseball—if it exists—is causally irrelevant to whether its constituent atoms, acting in concert, cause the shattering of the window.

(2) The shattering of the window is caused by those atoms, acting in concert.

(3) The shattering of the window is not overdetermined. Therefore,

(4) If the baseball exists, it does not cause the shattering of the window.

The rest of this chapter will, in one way or another, involve this argument, which I shall call 'the Overdetermination Argument'.

(Objects and Persons. Oxford University Press (2003), p.56)

For Merricks, the idea of the composite "baseball" can be excluded as overdetermining the shattering of the window. The analogy is powerful because the baseball is just an idea, just some information about the structure of the object, just its "form," like the form of a statue in the famous metaphysical puzzle of The Statue and the Clay.

The statue cannot survive the squashing of a lump of clay, but the lump can survive. Metaphysicians claim that the lump of clay and the statue have different persistence conditions.

Eliminative materialists deny the causal power of such abstract ideas or "forms." For them, only matter enters into causal relations. Form is separated from matter in many metaphysical puzzles and paradoxes. Form was imagined to be a numerically distinct object by the ancient Skeptics, but such pure ideas in minds are thought unable to move material.

Merricks' argument for the existence of humans goes well beyond that of van Inwagen. It brings up more subtle metaphysical problems and leads to some surprising conclusions, including the fact that humans have free will.

He begins by arguing that Kim's Exclusion Argument does not succeed in denying mental causation in humans! And his own Overdetermination Argument, based originally on Kim's Exclusion, also does not apply, because humans have causal mental properties that cause things that are not caused by our constituent atoms.

Sometimes my deciding to do such and such is what causes the atoms of my arm to move as they do. Presumably my so deciding won't ever be the only cause of their moving. There will also be a cause in terms of microphysics or microbiology, in terms of nerve impulses and the like. But at some point in tracing back the causal origin of my arm's moving (if it is intended), we will reach a cause that is not microphysical, that just is the agent's deciding to do something.

(Objects and Persons. Oxford University Press (2003), p.110)

Composite objects that cause things that their parts do not redundantly cause can resist the eliminative sweep of the Overdetermination Argument. We humans—in virtue of causing things by having conscious mental properties—are causally non-redundant. So the Overdetermination Argument fails to show that we do not exist. So I conclude that we do. For we should assume that we exist unless we are shown otherwise. Any conscious composita presumably survive the Overdetermination Argument just as we do. So I conclude that dogs and dolphins, among other animals, exist.

(ibid, p.114)

Human organisms do not dodge the Overdetermination Argument on a mere technicality of which baseballs, for example, cannot avail themselves for some intuitively irrelevant reason. Rather, human organisms have non-redundant causal powers and so can exercise downward causation. Baseballs, on the other hand, would not—even if they existed—have nonredundant causal powers or exercise downward causal control over their parts. This deep, fundamental difference between the powers of human organisms and the powers of alleged baseballs (and statues and rocks and stars and so on) makes all the difference with respect to the Overdetermination Argument.

(ibid, p.116)

Merricks' defense of free will is straightforward. He denies the thesis that "humans have no choice about what their constituent atoms do or are like." He says that

human persons have downward causal control over their constituent atoms. And surely downward causal control of this sort is sufficient for having a choice about what one's atoms do or are like...

On the assumption that we are human organisms, I have argued that we exercise downward causation...

I say that the downward causal control we exercise over our atoms makes room for our having free will. And, as we saw in the previous section, that same downward causal control undermines the Micro Exclusion Argument for mental epiphenomenalism. I think free will requires mental causation. So I think it bodes well for my metaphysics that its defence of free will turns on the same fact about humans as does its defence of mental causation.

(Objects and Persons. Oxford University Press (2003), p.159-160)

Merricks is correct that we have some downward mental control over some of our atoms, but wrong to suggest any control over what they "are like."

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