So isSolid is part of the trifecto, isLiquid and isGas? That doesn't seem to really serve any purpose, it should probably be renamed isTangible as that would mean we would have no use for it until the physics code for water and gravity exists. Same with isEmpty, if it means isGas then most usages are wrong, because gas can have a color and so it would be interacted with by the renderer.

So I am going to have to disagree with your definition of isEmpty and isSolid and say we probably use my definition, so we can still use the functions :D .