Which number episode of season 6 was the title "Sight Unseen"?
CREATE TABLE "table1_15431959_1" (
  "no_in_season" real,
  "no_disc" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text
);
SELECT MIN("no_in_season") FROM "table1_15431959_1" WHERE "title"='\"Sight Unseen\"';