Observation 1: A significant portion of humanity has expressed some feeling of spirituality. This has been expressed in radically varying ways across time, culture, and class. It is possible that many people were simply adopting the views of their culture’s traditions and felt nothing deep: that they would be non-feeling atheists if their parents and other associates were non-feeling atheists. But without doubt, authentic expressions of spirituality have been and are present throughout every division of humanity.
Observation 2: Many people talk about having spiritually fullfilling or spiritually unsatisfying lives, and this can change throughout the course of one’s life. Certain practices and outlooks can stimulate that fulfillment, and other practices can separate us from having it.
Naturalistic Assumption (or Induction): There are no spirits or spiritual entities or phenomenon in existence. (This is induced from a variety of arguments, that I am not presenting now. Therefore, this can be taken as having the weight of an assumption for the sake of this argument)
If one believes in the naturalistic assumption, and accepts the two observations (I think one must accept the two observations), then one must still offer an explanation for why Observations 1 and 2 exist. This cannot be called idiotic or delusional–since even if those were true, it would not be an explanation. Rather, there must be something about the private human experience with the world, and their understanding of their relationship with the world, that gives birth to both the ideas and the deep feelings.
Natural Causes 