Let me see if I understand this you are asking atheist to define our souls? In a world where pretty much every new nor ancient religion can't even come to a common ground about the definition of soul?
Isn't ….I dunno
….. define the souls subject a topic for religious people? And is up to the atheist to decide the logical and reasonable of their definition? Why it's our problem to define souls? Because if we are trading places then could you(a religious person I assume) define the influence, existence, and objective of the brains biochemical substances in a world where it's religious systems claims that it's an unquestionable fact that every emotion it's considered evidence of our spirits?
And no offense but if atheism is the lack of belief in any spiritual existence doesn't that answer the question by itself? Did it really needed a hole thread to answer that question? What did you expected we where gonna say?
Isn't ….I dunno

And no offense but if atheism is the lack of belief in any spiritual existence doesn't that answer the question by itself? Did it really needed a hole thread to answer that question? What did you expected we where gonna say?