I am working on my book report assignment and I have such sentence, I am positively sure that this book is a great book. My friend is editing it and comments, "positively" is not right to use here and he suggests I should either say, "I am pretty sure that ...", or "I am positive that ...". The reason is there is no such thing as "I am negatively sure".
I am wondering if he is right.
Could you let me know how you think?