It was about thirty foot away from the house, maybe, and I sort of got it into my head that, if I climbed up that tree, I'd very likely be able to see into that room.
I did not think I could see into the room if I climbed a tree.  
mnli hypothesis: I did not think I could see into the room if I climbed a tree.   premise: It was about thirty foot away from the house, maybe, and I sort of got it into my head that, if I climbed up that tree, I'd very likely be able to see into that room.