I am not a big Sodoku player. But I was having a conversation with a friend, about the game. My friend mentioned that in some of the very hard Sodoku puzzles you are forced to guess. We got into a disagreement. I say that all Sodoku puzzles can be solved by logic. My friend says otherwise. I was thinking of a way I can prove to my friends that I am right. I hope I am, but maybe not. After thinking about it for a few hours I came up with an argument. I'd like to know what others think, and if I am right or wrong. Basically I need input. Or if there is a better way to say what I am trying to below. Here is what I currently think: Suppose there is a Sodoku puzzle where you have to guess. If you work out the rest of the missing numbers through logic eventually you will figure out if your guess was correct or not. But by doing so you have logically concluded the correct number for the square, implying a guess was not needed. Say you have to guess with only 2 squares left, then the puzzle is not unique and thus not a Sodoku puzzle.