An alternating Turing machine is a non-deterministic Turing machine whose states are divided into two sets: existential states and universal states. An existential state is accepting if some transition leads to an accepting state; a universal state is accepting if every transition leads to an accepting state. (Thus a universal state with no transitions accepts unconditionally; an existential state with no transitions rejects unconditionally). The machine as a whole accepts if the initial state is accepting.
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“Psychiatric enlightenment has begun to debunk the superstition that to manage a machine you must become a machine, and that to raise masters of the machine you must mechanize the impulses of childhood.”
—Erik H. Erikson (1904–1994)