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As I understand it, there is basically hidden and public information in Magic. Now let's suppose I have multiple face-down cards on the battlefield, and my opponent gets to look at one of them. Is there anything in the rules that specifies what I can do to those face down cards and what I can't? My opponent now knows what one of my face down cards is, but does he have the "right" to know which one it is, or may I shuffle them so that he no longer has access to that information? My guess is shuffling is ok, but I have no idea where I'd look in the rules...

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There's a third kind of information: derived information, which is stuff that cannot be directly seen (like public) but which can be deduced from public information and the history of the game. If you (legally) get to look at hidden information (like what that face-down permanent is), it becomes derived information. Changing it would be changing the game state. –  Jefromi Sep 1 '13 at 14:54
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@Jefromi, That's not true. Whether something is private, derived or free has nothing to do with game state. It has to do with whether you must reveal that information to another player. e.g. Everyone could know the card on the bottom of your deck or the face-down characteristics of a card, yet it's still private information. You are never obliged to divulge or remind anyone of it. –  ikegami Sep 1 '13 at 19:51
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@Jefromi, "Derived information is information to which all players are entitled access, but opponents are not obliged to assist in determining and may require some skill or calculation to determine." e.g. The number of Goblins a player has is derived information. –  ikegami Sep 1 '13 at 19:52
    
@ikegami Because of the line "All characteristics of objects in public zones that are not defined as free information." I understood derived information to include this, because when a face down card has been revealed, its identity is a characteristic that all players have access to, and subsequently you can use your powers of deduction to say "aha, five seconds ago I saw under that card, and it's the same card, so I can deduce its identity". But I guess then it becomes private again. Moot point, anyway. –  Jefromi Sep 2 '13 at 21:52
    
@Jeromi, No player is entitled to know the characteristic of a face-down permanent. That line refers to the colour of a permanent, its type, etc Such information is derived as its affected by replacement effects. The only public characteristic of a permanent is its name. –  ikegami Sep 2 '13 at 22:07

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No, you can't do that.

From Comprehensive Rules (bold mine):

707.6. If you control multiple face-down spells or face-down permanents, you must ensure at all times that your face-down spells and permanents can be easily differentiated from each other. This includes, but is not limited to, knowing the order spells were cast, the order that face-down permanents entered the battlefield, which creature attacked last turn, and any other differences between face-down spells or permanents. Common methods for distinguishing between face-down objects include using counters or dice to mark the different objects, or clearly placing those objects in order on the table.

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That was really unambiguous, thanks! I know that this technically answers the question, but do you know is there something similar for other types of hidden information, e.g. the same scenario with looking at cards in hand instead of face down cards? –  Silly Freak Sep 2 '13 at 12:28

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