There seems to be an irreconcilable divide between the world of the physical and the world of the mental. It's so intuitive that the mind is distinct from the body, yet what is a mind without one? It's also so intuitive that life is distinct from non-life, yet this idea has been shown to be nothing but an artefact of the mind. At the core all life is chemistry, and the hard problem of life has dissolved in the last 100 years or so of scientific investigation.
Yet the mind doesn't have that apparent capacity of irreducibility, as David Chalmers puts it, there's the characteristic of what it's like to experience that cannot reduce to to the functional. So life can be seen as a different expression of the physical, but how does the mind reduce to that? This is the hard problem of consciousness, and one that won't go away any time soon.
So why am I a physicalist then? Surely the best position would be one of refraining from judgement until such time as a strong case has been made. And I think if the question were taken in isolation, this would be the case. But if we take the question seriously, it has to fit into the wider context of what we know about the world. After all, we are overwhelming surrounded by the physical, we don't know of any non-physical substance or force. What we're left positing is that there must be a unobserved stuff completely unlike what is physical, yet has the capacity to interact with the physical. It might be so, but we're left positing a complete new substance that itself is unexplained.
In addition to being composed of physical things, all interaction in our body points to purely physical measures. Our sensory material is all physical, information is sent through physical interactions, and processed in the physical brain, where again physical signals are sent through to cause physical movement. As far as what's observed, that's it. There could be something else involved, but there's no interface in the brain observed to show this.
Along this line, regions of the brain have been identified in what role they play in cognition. Through a number of lines of evidence this is well established. From observation of loss of function, through brain injury, to fMRI, to transcranial magnetic stimulation - there's overwhelming evidence that the brain is the fundamental component to cognition.
Then there's a comparative argument from nature. Other animals have similar behavioural functionality to us. In terms of what we can do, the difference seems to be the degree to which we can do it. If there's something to our mind beyond the physical, then do these other creatures share it? If not, do we need to posit anything more?
So what are we left not explaining? The first-person ontology of the mind, and that seems the hardest to overcome because its what we recognise as being the mind. But that leaves a huge problem to overcome. If physical accounts for everything causally, then what the mind is reduced to is not the mind as we know it. After all, we have the experience of thought to action. If the body does all that, then any sense of control we have is illusory.
We're left with an epiphenomenalist account where the mind is but a passive observer. But it gets worse, for if the mind is merely an observer of the body, then if there's no body then there's a cessation of experience for the mind. When our body ceases to function, what is there for the mind left to observe?
It's problems such as these that make dualism seem undesirable, in addition to the already problematic attempt to integrate the non-physical into our apparent physical reality. It seems odd to posit that it's all physical, but it's overwhelmingly where evidence and reason leads. The lack of a good physicalist account of a theory of mind, or qualia, or intentionality, are interesting questions for philosophers of mind to ponder over. But the absence of explanation or apparent reconciliation aren't really sufficient to ignore what the evidence overwhelmingly points to - that we are physical beings, and the mind is no exception.