Definition of "introspective"

All of a sudden Syme, who was standing as if blind with introspective thought, swung round and cried out, like a man waking from sleep—

1908, G[ilbert] K[eith] Chesterton, The Man Who Was Thursday: A Nightmare, Bristol: J[ames] W[illiams] Arrowsmith, […]; London: Simpkin, Marshall, Hamilton, Kent & Company, page 264