Ni seems to create a future concept of how they want things to be and look into it with the same conviction I would to a concept that is existing. They seem to say "Oh, I want a treehouse" and then once they have an idea of a treehouse, they can start waxing on the feeling of being in a treehouse. It seems a bit strange to me. They see what they are working towards as the "big picture". It doesn't seem to be bigger or smaller than my own picture, but where I consider understanding my own current information to be crucial, they consider it essential to understand their prediction of the future. To me it seems like a bit of a waste of effort, because the future is unreliable. Below I put my idea of a argument between very disagreeable versions of them (I apologize if I make it seem like Ni is worse or has worse arguments- it's just that I can create reasonable Si arguments much more easily than Ni ones because I am familiar with a Si line of reasoning):
Si: "How do you know what a treehouse is like?"
Ni: "I know what my treehouse is going to be like."
Si: "But how? You've never been in a treehouse."
Ni: "Well my idea of a treehouse has me feeling like X"
Si: "That doesn't make it true."
Ni: "Is your motto 'prove it' or something?"
Si: "No, I'm not saying that, I'm saying that you can't make observations about events that haven't happened."
Ni: "You can predict."
Si: "But you are talking about the future like it actually exists right now!"
Ni: "You are so stuck in the past!"
Si: "You think that the future is going to be one way just because
you think it is!"
Ni:

Si: