Eeh, if you take physics in
He is just energy, also same goes for his life cycles. Which means he would never truly be nothing and in the end be able to regenerate
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It never has, but it has only been stated that the doctor can be killed if he is killed mid regen. Which means that a shot from a lazer would vapprize him and his energy would still be existing and regenerating because of the time force. See: impossible girl, her body was turned into dust and materialized throughout the doctors life with help from the time force