I am reading the cornerstone book, "Artifical Intelligence, A Modern Approach" by Stuart Russel, and Peter Norvig and there is a passage in the book on page 98:
The complexity results depend very strongly on the assumptions made about the state space. The simplest model studied is a state space that has a single goal and is essentially a tree with reversible actions. (The 8-puzzle satisfies the first and third of these assumptions.)
What are the "assumptions" in that context?