In chapter one , "Of the nature of flatland":
very much like shadows — only hard and with luminous edges
What does he mean by "only hard"?
I just find it a little difficult to associate "shadow" and "hard", but maybe I am misunderstanding something.
Here's a longer quote to provide more context (emphasis mine):
Imagine a vast sheet of paper on which straight Lines, Triangles, Squares, Pentagons, Hexagons, and other figures, instead of remaining fixed in their places, move freely about, on or in the surface, but without the power of rising above or sinking below it, very much like shadows — only hard and with luminous edges — and you will then have a pretty correct notion of my country and countrymen. Alas, a few years ago, I should have said "my universe": but now my mind has been opened to higher views of things.
Best Answer
Here, “only” has the same meaning as “but”. The inhabitants of Flatland are like shadows, but they are hard and have luminous edges.
When he says “hard”, I think he means that two Flatlanders cannot occupy the same place at the same time, and they cannot change their shape.