Thought about this a bit more... Not sure if this is what you're saying, but the concept of "space between" I alluded to seems to arise naturally whenever you have ordered items, and vice versa. Because once you have order you have the concepts "greater than"/"less than", and once you have that you have a border between your items, and your items are between those borders. This connects back to Dijkstra's consideration of <, <=, etc....