Harry decides to change the past. He invents a machine and goes into the past and changes it. Now he can’t remember that he ever did that, in the old present that now never happened.
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Harry decides to change the past. He does, resulting in Harry’s no longer existing in the present. So he couldn’t have decided to change the past. He’s still here, in the present. Harry’s head hurts.
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Harry decides to change the past. He travels into the past, but then assumes he’s in the present. He decides to change the past. He travels deeper into the past, …
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Harry discovers that there is no past to change. “Past” is just a word for a special kind of present-time experience. This profoundly affects his thinking, and leads him to abandon science for cooking school.
Memorable Fancies 
Excellent. He’s probably better off cooking anyhow