Człowiek czy osoba?

To jest człowiek. To jest ważna osoba. 

This is a person. This is an important person. 

Or: On jest człowiekiem. On jest ważną osobą.
He is a person [in the sense of human being]. He is an important person.

"Portrait of Jakub Sobieski, castellan of Kraków", 

by unknown Polish painter, 1666. Oil on canvas. 

Lviv National Art Gallery.

I was considering the difference between człowiek and osoba, and also looked in the dictionaries, which were not very enlightening.

If you see someone, as above, you would say in English: "That is a person." In Polish, this would be: To jest człowiek. (Distinguishing him from animals or trees, etc.)

You can also use człowiek to express generalities: "A person never knows" would be in Polish "Człowiek nigdy nie wie." In this case, you would never use osoba.

When you start considering what traits the person has, you then can use osoba: To jest ważna osoba. Since you could also say To jest ważny człowiek, it seems that osoba also simply is a more elevated register.

I asked Szczepan, whose answer follows:

Człowiek jest pojęciem raczej abstrakcyjnym, używanym w sytuacjach "opisu ogólnego", kiedy opis odnosi się do wszystkich, bez względu na cechy osobowe, typu "Człowiek konsekwentnie dąży do celu", "los człowieka", etc.

Człowiek is [also] a more abstract concept, used in situations of "general description", when the description relates to everyone, regardless of their personal traits, such as: "people steadfastly pursuing their aims", "a person's fate", etc. [NB: The singular is usually used to express generalities in Polish, whereas in English - like here, with the word "people" - it is the plural that is used.]

Osoba - zwraca uwagę na cechy osobnicze, te, które ma opisywana "jednostka". "On ma bogatą osobowość". Pod tym pojęciem kryją się wszystkie, które wyróżniają jedną osobę od drugiej - walor personifikujący, ogólnie.

Osoba - emphasizes an individual's traits, those that are attributed to an "individual". "He has a rich [as in "complex"] personality [osobowość]." This term encompasses all the characteristics that distinguish one person from another -- something that makes someone the person he or she is, generally speaking.