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Whenever The Seventh Doctor attacks, choose a card in your hand. Defending player guesses whether that card's mana value is greater than the number of artifacts you control. If they guessed wrong, you may cast it without paying its mana cost. If you don't cast a spell this way, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
If an effect instructs you to choose a creature type, you may choose Time Lord.
If the card you chose has an {X} in its mana cost, the value of X is 0.
Neither "Time" nor "Lord" are creature types. Some older cards were printed with the subtype "Lord," but all of those cards have updated Oracle card text that removed that type.
To choose a card in your hand for The Seventh Doctor's ability, take a card in your hand and separate it from the others, face down. Then the defending player guesses whether the mana value of that card is greater than the number of artifacts you control. You do not have to reveal whether they guess right or wrong unless you are casting the card.
Unlike other creature types in Magic that are each only one word, the two words "Time Lord" represent a single creature subtype. Time Lord is the only two-word creature type.
