If a drug significantly alters your brain to the point that you no longer think the way you did before, how do you know you changed for the better? Maybe the old you, the you that existed before you took the drug, would say this is not better. Maybe the new you only thinks you are now changed for the better. Maybe you didn't change at all! After all, do you even have a way to measure what has changed, and how much better or not-better you've gotten? Do you just feel better? Much like an old dinged-up ball bearing with new grease, it may squeak less, but is the ball actually any different than it was before? And in the context of how we feel about ourselves, which really matters? Is our perception of ourselves more important than reality? If so, the statement "It has changed me for the better" is really just saying "I feel better than before".

Another way to look at the question "how do you know if the change made you 'better'": Say you took a drug, and suddenly you changed from being a liberal, to being a Nazi. (This happens a lot, not with drugs, but with events; an event happens, and suddenly someone turns from very liberal, to very far-right conservative) How do you know if the change has made you better? The new you is perfectly fine with Nazi-hood, and thinks you are better. But the old you would be pretty sure that the new you is the exact opposite of better. How can one tell if a change has made them better? Which version should we listen to? How do we define better, and how do we measure it?