From the introduction:
There is something mystical, even whimsical, about portable holes. Compared to their more common cousins, such as the bag of holding or gloves of storing, portable holes have a flavor and flair about them unmatched in the realm of extradimensional storage. Even the materials they are woven from are exotically ephemeral; silk of a phase spider, strands of ether, beams of starlight. Containers, on the other hand, make a solid sort of sense; they contain things. Holes, inasmuch as they can be defined as openings, apertures, or hollows, generally require containers. They can even be thought of as part of a container, as defining what makes a thing capable of containing something else. Thus, in contemplating the portable hole, it is perhaps best to bear in mind the sage advise that the Arch-Mage Corsicarius Larrandus, professor of theoretical geometry at Estenburg, gives to his students each year;
“Try not to think about it too hard.”