Spenta Mainyu: the Progressive, Life-Nourishing Mental-Force
Spenta Mainyu is the principle in reality which has created all the physical existence. A mainyu is best understood as a way of being in the mental realm which exists as both an option and a driving force for living beings. In this way, it can be seen as a broad mentality which exists outside any individual being but is taken upon by them if they chose to, thereby incorporating them as part of it. Zarathushtra seems to be using this non-individualistic conception of mentality to refer to driving, developmental forces which spur the growth (or malignment) of all existence. Spenta Mainyu is particularly the mainyu which progresses, grows, and nourishes all existence.
The meaning of Spenta is complicated, but it probably means ‘that which causes growth, nourishes’ with subcontext of holy. Full article coming soon.