What's fun is that you can invoke Humean relations to try to save persistence of identity over time, and preserve unseen entities. Imagine a pair of cubes in a void, each with sense-data of the other. So long as no other objects exist in the void, the cubes will retain their sense-data of each other, and thus the identity of each is preserved over time by virtue of being sensed by the other. Now imagine a wall between the two, but the wall has no sensory organs and cannot perceive or comprehend sense-data. Each of the cubes has a relation to the wall (as each cube has sense-data of the wall), with the wall having a relation to each cube (is perceived by each cube). This gets a bit hairy... if an object has no relation to any other, it exists in void. If an object is not in void, it must have a relation to at least one other object. Neither cube is in void; each cube can only have a relation to one other object, the wall. But because the wall has a relation to each cube, each cube can "know" that the other cube has persistent identity over time because the object they can perceive (the wall) maintains the relation to the unseen cube, and remains in each cube's sense-data. This (admittedly very rough sketch) argument is how you can demonstrate that if you have a chair in a room, leave the room, and come back, the chair you are perceiving is the same chair you put there (ignoring other potential factors).