Adverbials of viewpoint are detached elements in the sense that they are placed initially and separated from the sentence by a comma. They are ‘superordinate’, for they seem to have a scope that extends over the sentence as a whole. Their initial position is explicable for two reasons:
One is that viewpoint adverbials convey the speaker’s comment on the matter in what way or under what condition he is speaking as ‘authority’ for the sentence, or in what respect his statement is true or correct. So in ordinary speech and writing it is not uncommon to find some overt indication of authority accompanying the ‘Wǒ rènwéi…’ as a superordinate clause, as in ‘Wǒ rènwéi cóng zhèngzhì shang kàn, zhèige wèntí bìngbù jiǎndān’ (I think, if we look at the issue from the viewpoint of politics, it is not that simple). Hence, the viewpoint adverbial is, in fact, an adverbial element in a superordinate clause that is not overtly expressed.
Second, because adverbials of viewpoint specify the respect in which the statement of the clause that follows holds true, they function as a semantic frame, thus their semantic scope extends over the whole clause; hence they should precede the clause to which they apply. For the above two reasons, adverbials of viewpoint take the initial position of a sentence.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.