The comments show a lot of confusion about what Tesler invented. Other industries did indeed use cut/copy/paste and older editors had ways to do these functions. But the cursor in these editors normally indicated a character. Larry figured out that if instead the cursor indicated the space between characters and he sometimes had a second such cursor to indicate all characters between them then he could do what previous editors needed various commands with a single operation: replace the selection with what has just been typed and move the cursor to right after that. "paste" would the just the equivalent of retyping a previous selection that had been either "cut" or "copied".

If the two cursors were at the same spot (just a blinking vertical bar) then you are inserting text as you type it. If there was some selected text then you are replacing it and then inserting anything more you type. And so on.

Both at Xerox Parc and at Apple he actually tested his ideas on potential users and often found he guessed wrong about what would work and what wouldn't. He would then try something else.