I can't understand what is the role of word "one" in this sentence:
A hillside in the morning so radiant with light one has the feeling he has been looking at Van Gogh's drawings.
Source: The Guardian
I know that it says the hillside is very radiant such as Van Gogh's drawings. But I don't understand what role word "one" grammatically has and what does it mean in the sentence.
Thanks for any help.