The book will be especially rewarding if you're looking for an answer to either of the two following questions: what are the origins of the Japanese emperor system and what are the origins of the discrimination of outcasts?
The title of tennô is of course interesting to interpret in the light of Ooms' thesis on the centrality of Daoism in the politics of this period. The title may have been attractive to Temmu for many reasons: unlike the title "king" (ô or wang) it didn't imply subservience to the Chinese emperor, and unlike older Japanese terms like ôkimi ("great king") it clearly suggested divinity. Rather than referring to the shintô Sun Goddess, however, this divine element referred to the Daoist God of the Polar Star or Dipper, the Supreme Heavenly Ruler who was thought to be controlling yin and yang. The association of the ruler with the Polar Star is an old one in China, and can be found already in the Analects. In the Han era, the pole star was used as a metaphor for the ideal of the passive ruler, who does little but radiate virtue but nevertheless functions as the central hub around which everything moves - "sitting on the throne of non-action and riding on the perfection of his officials”, in the words of the court scholar Dong Zhongshu. The identification of the polar star with Tianhuang Dadi also dates from Han texts, and during the mid-second century AD, the belief took root that earthly human beings could acquire the shape of this deity. Finally, the third Tang emperor Gaozong – a contemporary of Temmu – changed his title from huangdi to tianhuang (tennô). Although the possibility exists that Gaozong might have inspired the Japanese tennô, Ooms believes - like Piggott - that older Han texts were a more likely source. Here his opinion differs from that of Tim Barrett, who in a recent study ("Shinto and Taoism in Early Japan", in J. Breen & M. Teuwen, eds., Shinto in History: Ways of the Kami) argues for influence from Gaozong.
Finally, Daoism also contributed to the political valorization of "purity" which started during Temmu's reign and provided the ideological justification for dividing the population into "good" and "base" (ryômin and senmin as stipulated in the Taihô Code). ”Purity” was a central value in Temmu's legitimation project which was evident in the naming of court ranks and in the name of his palace, Kiyomihara (Pure field). The discrimination of the ”defiled” in Japan can thus be traced back to Temmu.
Ooms is not claiming that internal power struggles (the shift from Tenji to Temmu) alone explain this new set of imperial symbolics. What superficially appears as a vainglorious attempt at competing with China by claiming peer status as an "empire" in its own right (as indicated by terms such as "all under heaven" or "central kingdom") is explained both by internal factors – that Temmu had to legitimize his rule after usurping the throne – and external factors – that the threat from the new and vigorous Tang empire provoked centralization and state building in Korea and Japan. "Nippon" was thus born from the unification of China and from the combined Tang and Silla attacks on Paekche, which forced the Japanese rulers to strengthen their own state.