Essentially, the moment you identify is one where Montag is literally poised between two worlds. The first world is the one that is in pursuit of him. This world is punctuated by the mechanical hound, the helicopters above him, and the idea that his past seeks to extend one more instance of hold over him. Montag is "moving from" this vision of reality, an "unreality" because he has recognized its false and contrived nature. For so long, he believed this to be reality and in its flight, Montag flees from himself. Yet, at the same time, Montag is in pursuit of a world that is unknown to him. It is here where Bradbury is really quite profound. He recognizes that it is slightly artificial to depict Montag running away as something that is easy or without pain or difficulty, or even doubt. Montag is leaving a condition of life that had constituted reality for so long. Although it is now "unreality" it still represents the only reality he had known. For him to literally be in flight from it is "frightening." It is filled with fear because of its "new" state. This newness carries with it the unknown and that which is uncertain, contributing to the anxiety associated with it. When the machines and hound stop pursuing him, "as if they had picked up another trail," Montag is surrounded by only nature, a state that is new to him and one that is uniquely different from anything else he had experienced. It is here where there is fear and anxiety because of its new state. While this is present, it is not something that will hold Montag back, as it actually feeds his desire to move ahead, filled with fear, insecurity, and anxiety. The "murmering ghosts" of the past are now leaving him, with only "newness" in front of him. For this, there is a natural condition of fear.