I've re-read your post and tried to understand what Mr. Harris is trying to say. His definition of spiritual is his definition of spiritual. You can also find it in various dictionaries, almost all of which use words such as non-material, supernatural, soul, having the nature of a spirit. My point, which I believe is relevant to the overall topic and OP, is that spirit/spiritual/spirituality are primarily associated with the unseen god-like forces common in religion. - DJS
Spirituality is the praxis and process of personal transformation, either in accordance with traditional religious ideals, or, increasingly, oriented on subjective experience and psychological growth independently of any specific religious context. In a more general sense, it may refer to almost any kind of meaningful activity or blissful experience. There is no single, widely-agreed definition for the concept, and the term may be applied to a wide variety of practices.
http://en.m.wikipedia.org/wiki/Spirituality