we can never know our own true reason for doing something
I read Sam Harris’s book on free will which I think is what you’re referring to, but I don’t recall him saying anything like that. If he did, I presume he meant something like “you don’t know which set of physical inputs to your neurons caused your neurons to fire in a way that caused your behavior”, which doesn’t mean you can’t have a belief about whether someone’s motivation is (say) religious or geopolitical.
I read Sam Harris’s book on free will which I think is what you’re referring to, but I don’t recall him saying anything like that. If he did, I presume he meant something like “you don’t know which set of physical inputs to your neurons caused your neurons to fire in a way that caused your behavior”, which doesn’t mean you can’t have a belief about whether someone’s motivation is (say) religious or geopolitical.