But you've just made that up though, If that was a true experience then if would be a different matter.
Now what I want to get to is the following,
Person (1) "In my experience I have not seen any negative effects with the use of cannabis"
Person (2) "I have seen first hand on a number of occasions where cannabis has effected people negatively"
Now assume that these are normal respectable people that are telling the truth in regards to their experiences, now person 2 has observed something person (1) hasn't, these experiences don't equal each other in terms of proving a point, both experiences are valid of course, but one has experience of something the other person hasn't, which of course makes sense becuase not everyone is going to be effected by cannabis negatively, now if person 2 is telling the truth then their experiences at least carries more weight into the debate becuase they have new observations to enter into the equation that need to be addressed, the question is though can person 2 prove the connection between cannabis and mental health disorders scientifically, that's the hard part.
We're not talking about the existence of God here, the connection between cannabis and Mental health issues isn't a faith or religion that has been the foundation of many cultures for thousands of years, this is something relatively new, more and more 'normal' people, doctors, psychiatric nurses etc are starting to see a connection between the two, but it's not something that can be easily proved atm.
Hell we can't even prove conclusively that people are born gay yet, but then I assume many people on here think people choose to be gayers or it was a product of their upbringing that can be reversed by the local priest or shrink.