One traditional approach to the mind-body problem in philosophy is this: look for some neutral stuff. The interaction of an immaterial consciousness with a material body/brain would seem less mysterious were there some neutral stuff out of which both consciousness and physicality were themselves created.
The term "neutral" here means simply that the theorist at issue isn't trying to dissolve one half of the dichotomy into the other.
There are three broad types of neutralism, one of which is simply called "neutral monism." The other two that one can put into this category have different names and slightly different approaches: panpsychism, and double-aspect theory.
I will say a brief word now about how each of them relates to the philosopher who inspired the existence of this blog: William James. Tomorrow, I'll try to say something about how they are different.
William James is a and perhaps even the key figure in the development of neutral monism narrowly understood, though I'm not sure how seriously he wanted us to take it. The "New Realists," like Holt and Perry, took these ideas directly from James, and were entirely serious about them. The idea is that one can speak of the sight of a piece of paper as a fact (sight-per, let us call it). The sight-per considered in itself isn't anyone's sight in particular, and it isn't any particular piece of paper in particular. As a datum in a substratum it has not yet become part of any story.
For various purposes, the sight-per does become part of various stories. It may be the story of my life. In that case, it becomes MY visual datum, and evidence of the existence of an item sitting on top of MY desk., Or yours.
But you and I and the desk are all composed of this stuff, data like sight-per, and data like THAT are the desired neutral ground.
Panpsychism is the view that a sort of psychically-understood matter is the neutral stuff. The piece of paper itself has a mind, or is a mind, as well as being a physical object. This conviction sometimes develops in stages -- a thinker may decide first that he cannot countenance a distinction between animals and plants, so that if the former are mindful the latter must be too. But then, as one philosopher (Lotze) put the point, there is a temptation to go further. Lotze wrote, "One cannot search for the mind arbitrarily in the plants, the darlings of our fantasy, and remain satisfied with the existence of dead matter in the rocks."
Lotze in writing that comment was criticizing Fechner for only making half that journey -- Fechner had assigned mind to the plants but not the rocks. Fechner later in effect conceded the Lotzean point by going the whole way into panpsychism,. James offered a sympathetic summary of Fechner's development in a chapter of his book A PLURALISTIC UNIVERSE.
Those two sorts of neutralism are very different from one another. Neutral monism is from one point of view a ruthless application of Ockham's razor. Panpsychism rather jubilantly throws the razor away and grows a beard, then declares the beard a living and mindful creature.
Dual Aspect Theory
Dual-aspect neutralism is, in tone, somewhere in-between the other two neutralisms.
As a historical matter, we may go back to Spinoza or, if we do some back-testing, even to Parmenides. The key thought here is that there is only one Substance in the world, and that mind and matter are different aspects of that One, so that physics studies the psychology of God.
THIS, to William James, is anathema. It is the Block Universe, it is deterministic, it pre-figures Hegel's Absolute. James once shouted, "Damn the Absolute." He meant by Absolute THIS sort of conception of God. So it seems to me we have a Jamesians have a bit of a paradox on our hands, philosophical as well as exegetical. If we see these neutralisms as a spectrum, with dual aspect theory the mid-point on the spectrum, then we'll have to think of James as playing along with both ends, but as utterly averse to the middle. Why so? I hope to expound it a bit further tomorrow.