Ah. I didn't understand that "that" was the argument. In that case, you "still" have all your work ahead of you. "No," a typical western atheist would not "assume unseen qualities in a person" by assuming that person has a "mind." You would have to demonstrate that "mind" is immaterial first, but I doubt you'd get much help from neuroscientists who can pinpoint areas in the brain that control emotions, thoughts, etc. At the very least, we can demonstrate that "mind" is physical. But you cannot demonstrate (apart from analogy) that mind is immaterial. Even if you could demonstrate or prove that "mind" is immaterial, it still doesn't prove a spirit exists, and it still wouldn't prove God exists. Can you: 1) Prove or demonstrate that "mind" is immaterial (apart from anaolgy and metaphor)? 2) Prove "spirit" exists, apart from analogy and metaphor (eg "...the spirit is like the wind...")? Again, for a Christian, these things are assumed and taken on "faith," but if you're trying to offer proof or evidence for such immaterial entities, you'll come up short because we have no reliable "test" or "mechanism" to demonstrate something like a spirit existing. Ultimately, an atheist does not have to "believe in unseen realities" in the way that you mean them. They do believe in unseen phenomena, such as someone on the other end of the cellphone and that you experience feelings that they cannot directly see, but this, again, is inline with our experience of interacting with the physical world and other humans. It does not prove that "minds" are immaterial or that spirits exist.