Another question, Clive. If you were supposed to make a general statement on experts, which would it be your choice and why? (1) Every expert has his knowledge of his own fields. (2) All experts have their knowledge of their own fields.
I think both sentences sound awkward, but if I had to choose one, I'd choose (2) after removing the first 'their': (2) All experts have knowledge of their own fields.
My preference would also be to use 'in' rather than 'of' (i.e. in their own fields).
«Every expert has their/his own level of competence in his/their field» or, in other words, «For any given fileld of science (or whatever you mean), level of competence differs greatly among experts»
Is that the thought you want to express? If it is so, then I am afraid neither Clive's nor Yankee's re-wordings work
1. All men have a head 2. Every man has a head Of, course, these sentences have literally the same meaning, but, as Clive and Yankee, I see a subtle difference in the perception of "all" and "every". "All" is like uniting everybody in one big group (set) and then making a statement about this group.
"Every" is more like enumerating the items of this grout o