No, I don't think you understand what I'm trying to get at. Clarke describes the world as being flat, because of the high gravity. There are no hills or mountains. Its inhabitants are flat (OK, two dimensional). And the question Clarke asks is interesting. Think about it. How often do we express ourselves in terms of height? Things like hierarchies, pay rises, position, etc. Or, consider what happens if one of these creatures is suddenly rotated through ninety degrees - does that creature still exist as seen by other creatures of their sort? Or has the rotated creature vanished? If the vast majority of your species don't or can't understand the concept of a third dimension, how do you express things? If youhave a belief system, what does it look like? How would you translate concepts from your language to those of another species who do think in three dimension? Our concepts reflect what we see around us, but they are formed by how we express those concepts in our everyday language. The fact that we think and express things in terms like compassion doesn't mean that other species out there would understand what we mean. Yes, they might have similar concepts but they might express them very differently and we might not understand each other when we try to translate our concepts.Of course they would understand a third dimension. Just because they themselves are flat, doesn't mean that something around them (rocks or low mountains) wouldn't have depth. If they are intelligent creatures, they would understand this simply by looking around, or if they can't see, feeling the way the elevation changes. While I haven't read that book specifically, who's to say they don't have an emotional connection to each other? Just because one can sit on top of another, or beneath another doesn't mean that isn't a form of intimacy in itself. Humans do it all the time. Snuggling together for warmth, for company, laying/leaning on one another at the back of a classroom just for companionship. Touch is probably one of the most intimate things Humans can experience. Why wouldn't the same be true of some 2D aliens on a distant world?
Just a question, having not read that book: Were those 2D alien technologically advanced, or just some critter Clarke used for window dressing to describe a planet such as the one in the book?