You one day receive a knock on the door, you go and open the door. When you look who it was, you see a split second of a figure walking away, but on the ground is a book. And that book it shows you instructions on how to make a time machine step by step manual. You go to your workshop and start working on it and finally make it. And the book it says after you made the machine that you must go back in time and give yourself this book. You do so. Go back in time, find your house, knock on the door, leave the book on the floor and walk away. The question is, who left the book on your doorstep? When you're the one leaving it to yourself now, who did they get it from? Where did the book originate first?