Were I an atheist, I would be a physicalist. That is, I would believe that reality - every bit of it, to include ideas, hopes, dreams, fears, loves - is only physical. Reality is particles, without remainder.
Were I an atheist, physicalism would present some problems for me. One problem would be the "Mary argument."
In 1982 Australian analytic philosopher Frank Jackson presented his "Mary argument" against physicalism. Jackson wrote:
"Mary is a brilliant scientist who is, for whatever reason, forced to investigate the world from a black and white room via a black and white television monitor. She specializes in the neurophysiology of vision and acquires, let us suppose, all the physical information there is to obtain about what goes on when we see ripe tomatoes, or the sky, and use terms like ‘red’, ‘blue’, and so on. She discovers, for example, just which wavelength combinations from the sky stimulate the retina, and exactly how this produces via the central nervous system the contraction of the vocal chords and expulsion of air from the lungs that results in the uttering of the sentence ‘The sky is blue’.… What will happen when Mary is released from her black and white room or is given a color television monitor? Will she learn anything or not? It seems just obvious that she will learn something about the world and our visual experience of it. But then is it inescapable that her previous knowledge was incomplete. But she had all the physical information. Ergo there is more to have than that, and Physicalism is false." (In "Qualia: The Knowledge Argument.")
Did Mary learn something new? Did she have new knowledge? Jackson, and others, believe the answer is yes. (See, e.g., "The Ghost in the Machine: Embodied Souls," by Stewart Goetz. In Loving God with Your Mind: Essays in Honor of J. P. Moreland.)
The Knowledge Argument Against Physicalism is this:
(1) Mary has all the physical information concerning human color vision before her release.
(2) But there is some information about human color vision that she does not have before her release.
(3) Therefore, not all information is physical information.
Were I an atheist, this would trouble me. My experience of troubledness would resist reduction to physical particles.