The book contains techniques that some people may find useful.
Can anyone give me some explanation on this sentence whether it's a fact or opinion.
Seems like a very basic question, but it really makes my head spin. For me, this is definitely a opinion. I understand that statement like "Tina Goodman said that music helped improve her health." is a fact if she did say it, despite what she said is opinion. But this sentence isn't something like "He believes the book contains techniques that some people may find useful". In addition, the "some people may find useful" modifier signify that a sentence refered to such techniques isn't always true; they may or may not be useful to people. However, one stated otherwise. So I'm confused.
Thanks in advance.