What is the affinity between ghosts and the ethereal?

Question

In D&D cosmology, we learn that the Ethereal connects the Material to the Inner Planes (Elemental, Feywild, Shadow, etc) and that the Astral connects to the Outer Planes (Gods and afterlives stuff). We also learn that a ghost is a soul that is 'tethered' to the Material for some reason or another to prevent its trip to the afterlife.

So why are they the most populous and iconic inhabitants of the space between mortality (Material) and ....fire salamanders (Elemental)? Is it easier to travel from Material to Ethereal to Astral to the Outer Planes, and thus that's the way dead souls are 'sucked'? The ability to see the living seems useful to a ghost and there have been vague references to the ectoplasm of the Ethereal being connected to 'memory' somehow (which might be thematic if it wasn't so vague), but why is the 'backstage' where raw materials are turned into the Prime Material the iconic place where post-material souls hang out instead of the Astral?

Answer 1

It’s simply because anything that is present, but incorporeal, on the material plane by definition exists partly in the Border Ethereal. An entity like a ghost, that is tethered to the material plane but has an incorporeal portion, has that part in the plane that is — definitionally — where incorporeal parts of material plane entities exist.

That’s the role of the ethereal plane: being the transition plane between the material and everything else in the planar cosmos. It isn’t primarily the place where “raw materials are turned into the Prime Material”: it only happens to also be that because its foremost nature is being the medium through which the inner planes connect, in both directions.

By contrast, the astral is not adjacent to the material plane, so it’s not a candidate for ghosts’ incorporeal portions. Travelling to the astral plane “naturally” — without portals or spells that take you there — requires passing through the ethereal first. Ghosts are trapped in the border ethereal and can’t even reach the rest of the ethereal plane, because they can’t fully leave the material plane. They certainly can’t reach edge of the ethereal where it meets the astral, and its access to an afterlife in the outer planes.

Answer 2

Insubstantive ghosts live in the insubstantive Ethereal plane; although historically, the Negative Plane which powers undead was considered an Inner Plane connected to the Ethereal.

The D&D 5e Dungeon Master's Guide, p.49, merely notes that ghosts are Border Ethereal, rather than being natives of the Deep Ethereal; they are closer to being natives of the Material who simply possess the ability to phase out:

Most encounters in the Border Ethereal are with creatures on the Material Plane whose senses or abilities extend into the Ethereal Plane (phase spiders, for example). Ghost also move freely between the Ethereal and Material Planes.

While D&D 5e's DMG on page 43 suggests that the Positive and Negative planes are described as a separate category to Inner Planes, which is a term that seems to refer to the elemental only, this wasn't the case in earlier editions of the game. The ghost's connection to the Ethereal dates back to these earlier editions.

For example, The Inner Planes (Dragon #73) considers Positive and Negative along with the four Elementl planes to account for six Inner planes, overlapping in some areas to create four Para-Elemental planes and eight Quasi-Elemental Planes. D&D as late as third edition considered this layout to be authentic (e.g. including game stats for quasi-elemental mephits), and the removal of Positive/Negative from the Inner Planes is a more recent change.

In that regard, the Negative Plane is traditionally connected to the Material Plane via the Ethereal.

However, there's no particular canonical reason in D&D 5th edition material to state that a ghost draws any kind of energy or sustenance this way. The only real statement about the ghost is that it is a creature of two worlds, Material and Ethereal, and can move freely between them. It may simply be a convenient transitory plane for the ghost to hide in.

The idea that ghosts hang out in the Ethereal Plane dates back at least as far as Dragon #42 (Oct 1980), where it appears in a random encounter table for that plane. In fact, the earlier AD&D Monster Manual describes ghosts as "non-corporeal (ethereal)" and only capable of maninfesting in a semi-substantive form on the Material, suggesting that they live in this hazy plane and are themselves hazy.

One might argue that the ghost moves into the Ethereal toward the Inner Planes to avoid its natural fate, which would otherwise see it pulled in the opposite direction toward the Outer Planes. However, that would largely be speculation.