The existence of God ranks among the most important mysteries that anyone can explore. Countless arguments have been made for and against God's existence over the course of human history and, despite centuries of debate, rational disagreement still remains. In light of this disagreement, it is tempting to suppose that we should refrain from drawing any strong conclusions on the matter. The lack of a belief in the existence or non-existence of God, otherwise known as agnosticism, is a position I am inclined to reject. While I do not claim that my view is the only one a reasonable person can hold, I am convinced that there are persuasive reasons to think that atheism, the belief that God does not exist, is true.
Traditionally, atheists have appealed to the existence of evil or the apparent hiddenness of God in order to make their case. While I certainly think that there are forms of these arguments, such as the evidential argument from evil, that have substantial merit, my primary reason for rejecting theism rests upon the extraordinary power and coherence of an atheistic worldview. Specifically, progress in the empirical sciences has given us tremendous evidence that the concrete, particular things that make up our world are entirely physical. Furthermore, the arguments raised against this claim strike me as unsuccessful.
Physicalism, the view that every concrete thing is physical, is supported by our discovery that the things we encounter in our world have all turned out to be physical things. Tables and chairs have turned out to be ultimately realized by microphysical particles, along with rocks, plants, oceans, and even ourselves. That last claim is a bit more controversial, but upon reflection, I believe it can be sustained.
One apparent difficulty for the view that we are material beings stems from the peculiar nature of our conscious lives. It seems absurd to suppose that our pains, itches, emotions, and sensory experiences can all be understood as features of the brain. However, when we focus on the way these mental phenomena influence our behavior, its hard to see how it could be otherwise. For example, we know that our pains are what cause us to move in certain ways. When we burn our fingers, we pull our hands away from the heat. But neuroscience has provided a complete explanation of this reaction in terms of activity in our brains. If our mental lives are distinct from brain activity, science gives us a reason to believe that they do not account for our behavior at all.
But we are not forced to draw any such conclusion. Given that our mental lives influence our behavior and, as science shows, our behavior is fully explained by physical activity in the brain, the natural conclusion to draw is that our mental lives and the activity of our brains are the very same thing. This conclusion is no different from the discovery that heat is identical to certain sorts of molecular motion or the discovery that water is H20. And just like these discoveries, we can reasonably presume that they apply universally. Science gives us reasons to believe that all heat is molecular motion, all water is H20 , and all mentality is physical.
These discoveries reflect the general picture motivated by the natural sciences concerning the world as a whole. Empirical investigation has consistently revealed a physical foundation underlying everything found in nature. This achievement, in turn, supports a worldview according to which physical reality accounts for concrete reality as a whole. In the absence of evidence to the contrary, we have evidence for materialism that is, to my mind, very powerful.
The implications for theism are relatively straightforward. Given the plausible assumption that God is a concrete, immaterial being, science provides us with evidence that God does not exist. Of course, this is far from the end of the conversation. As I mentioned earlier, many intelligent individuals have provided arguments in favor of God's existence and against the truth of physicalism. A complete discussion of these issues would be necessary to arrive at any confident judgment on the matter. My purpose has not been to explain my reasons for rejecting theism. I only hope to have clarified one positive reason for being an atheist. It is the most compelling one I have found.
[Addendum - My specific definition of physicalism would be something like the following:
Physicalism is true iff 1. Any world that is a minimal microphysical duplicate of the actual world is a duplicate simpliciter, 2. All higher level concrete phenomena are constituted/realized by lower level, physical phenomena, and 3. There is no fundamental mentality. I define "physical phenomena" as any concrete phenomena identical to or ultimately realized by the posits of an ideal microphysical theory ("ultimately realized" is understood straightforwardly in terms of transitive realization). There are substantive questions about how to properly formulate the physicalist thesis but this seems sufficient for my purposes. Ultimately, however, I suspect the best formulation will include reference to metaphysical ground.]
