original_query: What is the main thesis presented by Arthur Koestler in his book "The Thirteenth Tribe"?
query: What is the main thesis presented by Arthur Koestler in his book "The Thirteenth Tribe"?
answer: Koestler's main thesis is that Ashkenazi Jews are not descended from the ancient Israelites, but rather from the Khazars, a Turkic people who converted to Judaism and migrated westward.