This comes from Herbert Giles' (1845-1935) "China and the Chinese", which was a series of lectures given at Columbia University in 1902.
"Theoretically speaking, the Empire of China is ruled by an autocratic monarch, responsible only to God, whose representative he is on earth.
Once every year the Emperor prays at the Temple of Heaven, and sacrifices in solemn state upon its altar. He puts himself, as it were, into communication with the Supreme Being, and reports upon the fidelity with which he has carried out his Imperial trust."
"And just as the Emperor is responsible to Heaven, so are the viceroys and governors of the eighteen provinces-to speak only of China proper-nominally responsible to him, in reality to the sex departments of state at Peking, which constitute the central government, ..."
Elsewhere, it appears clear that there can only be one legitimate ruler and one empire, because of this unique relationship between the emperor and heaven.
This generates some curious conclusions as we fast forward to our current era. For example, the communists are technically atheists, so on what basis do they derive a "one China" principle? As for Taiwan, their current leaders seem to be too muddled in their thinking to contemplate such lofty notions as a Heaven and how their authority might be related to it.