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The Tenth Doctor & the Eleventh Doctor are different incarnations of the same Time Lord. When one incarnation of the Doctor regenerates & becomes another incarnation, his appearance changes & so do many aspects of his personality but he remains the same basic person & retains his memories. He may, however, suffer temporary amnesia and/or other forms of instability until the new incarnation "settles in". Both the Tenth Doctor (in "The Christmas Invasion") & the Eleventh Doctor (in "The Eleventh Hour") experienced some degree of this regeneration trauma. The Tenth was seriously ill for a time, whereas the Eleventh suffered only minor effects.