Knights and Knaves

 
Knights and Knaves

Problem statement:

There are three people (Alex, Brook and Cody), one of whom is a knight, one a knave, and one a spy.

The knight always tells the truth, the knave always lies, and the spy can either lie or tell the truth.
 
Alex says: “Cody is a knave.”
Brook says: “Alex is a knight.”
Cody says: “I am the spy.”
 
Who is the knight, who the knave, and who the spy?

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Alex is a Knight.
Brook is a Spy.
Cody is a Knave.
 
Brook is not the knight, since if he is, then Alex would also be the knight.
Cody is not the knight, since his statement would then be a lie.
 
Therefore Alex is the knight. Hence Cody is the knave, and Brook is the spy.