If God is in fact a living determinate being, and is outside the natural system, he might insert events into the natural system. The laws that we observe in the natural system may be a subset of higher laws that govern the universe. What criteria do you use to determine if a miracle has taken place? Evidence plus intrinsic probability. Whether or not an event is a miracle is also part of the discussion of the problem of evil. Why would God intervene in some circumstances but not others?