[Lazarus] Are private data not allowed? Why?

[Massimo Soricetti]

I would like an explanation on this code (noob question, maybe):

private
   sDati: array[0..3] of string;
   procedure Verifica(var: sDati array of string);

This code returns an error because sDati is already declared. If I move 
the declaration in public space is OK:

private
   procedure Verifica(var: sDati array of string);

public
   sDati: array[0..3] of string;

why this? Private data are not allowed? Why?