It has been observed by the curious, and beautifully described by Mr. Addison and Mr. Locke, that in the scale of beings, there is such a gradual progress in nature, that the most perfect of an inferior species comes very near to the most imperfect of that, which is immediately above it: that the whole chasm in nature, from a plant to a man, is filled up by such a gentle and easy ascent, that the little transitions from one species to another are almost insensible: That if the scale of beings rises by such a regular progress so high as man, we may, by a parity of reason, supppose, that it still proceeds gradually through those beings, that are of a superior nature to him; that there is no manner of chasm left, no link deficient in this great chain of beings.
Now according to this observation, which is apparent through all the known works of God, and by a parity of reason presumed of those above our knowledge, there should be in nature some being to fill up the vast chasm betwixt body and spirit; otherwise the gradation would fail, the chain would seem to be broken. What a gap between senseless material, and intelligent immaterial substance, unless there is some being, which, by partaking of the nature of both, may serve as a link to unite them, and make the transition less violent? And why may not space be such a being? Might we not venture to define it, an immaterial unintelligent substance, the place of bodies, and of spirits, having some of the properties of both.
[Catharine Trotter Cockburn, "Remarks on the Notes by Archbishop King's Translation Concerning Space, &c., With a digression on Dr. Watt's Notion of Substance," in Catharine Trotter Cockburn: Philosophical Works, Sheridan, ed., Broadview Press (Toronto: 2006) p. 97.]
Cockburn is one of the too-often-overlooked great philosophers of the early modern period. This passage is from an appendix to a work published in 1743. Space as a substance intermediate between mind and body makes for a somewhat striking picture, a version of a view that was actually quite popular in the early modern period; to accept it you would have to have very particular assumptions, but I imagine that a poet or fantasy writer could do some splendid things with it.
(I intended to note that the reference to Locke is to ECHU III.iv.12 and IV.xvi.12.)