Spirit is a substance
The strongest direct reference to spirit as a substance may be [30:1.13]:
There are spirits: spirit entities, spirit presences, personal spirits, prepersonal spirits, superpersonal spirits, spirit existences, spirit personalities—but neither mortal language nor mortal intellect are adequate. We may however state that there are no personalities of “pure mind”; no entity has personality unless he is endowed with it by God who is spirit. Any mind entity that is not associated with either spiritual or physical energy is not a personality. But in the same sense that there are spirit personalities who have mind there are mind personalities who have spirit. Majeston and his associates are fairly good illustrations of mind-dominated beings, but there are better illustrations of this type of personality unknown to you. There are even whole unrevealed orders of such mind personalities, but they are always spirit associated. Certain other unrevealed creatures are what might be termed mindal- and physical-energy personalities. This type of being is nonresponsive to spirit gravity but is nonetheless a true personality—is within the Father’s circuit.