In fact, every major male character in the story is Bob Wilson at different stages of his life, traveling back and forth through the Gate. Even his own origin is circular: the knowledge and means to build the Gate come from his future selves. He is, literally, the author of his own existence within the time loop.
However, with each jump, Doc realizes that his actions are having unintended consequences. His younger self is becoming increasingly dependent on the advice and guidance of his older self, and Doc begins to question whether he is truly helping himself or creating a paradox.
This story is the seminal literary example of the "Bootstrap Paradox" (or Causal Loop). Heinlein poses the question: Where did the knowledge come from? Diktor gives Wilson a notebook of vocabulary; Wilson copies the notebook to give to his younger self later. The notebook exists without ever being written. Heinlein treats time not as a flowing river, but as a static landscape where everything has already happened.
Fans of time loops, hard science fiction, logic puzzles, and the "Grandfather Paradox."
Thus, Bob’s entire adventure—including his struggles, his lovers, his rivalries, and even his aging into the Ancient One—is a closed, self-consistent loop. Nothing is “changed” in the past; everything always happened exactly this way.
