I groaned with horror, doubting whether I was not out of my senses; but the Stranger continued: Surely you must now see that my explanation, and no other, suits the phenomena. What you call Solid things are really superficial; what you call Space is really nothing but a great Plane. I am in Space, and look down upon the insides of the things of which you only see the outsides. You could leave this Plane yourself, if you could but summon up the necessary volition. A slight upward or downward motion would enable you to see all that I can see.
Chapter 17. How the Sphere, Having in Vain Tried Words, Resorted to Deeds - Flatland: A Romance of Many Dimensions (1884) - PART II: OTHER WORLDS