"Cannabis use appears to be neither a sufficient nor a necessary cause for psychosis. It is a component cause, part of a complex constellation of factors leading to psychosis."
Arseneault, Louise; Cannon, Mary; Witton, John; Murray, Robin M. (2004). "Causal association between cannabis and psychosis: examination of the evidence". The British Journal of Psychiatry 184 (2): 110–117. doi:10.1192/bjp.184.2.110. PMID 14754822.
http://bjp.rcpsych.org/cgi/content/full/184/2/110.
That being said, there is evidence to the contrary. The mind is a complex place that no one fully understands. Weed could be able to access a part of you mind that usually isn't acknowledged, thus leading to the "mental illnesses." My aunt was schizophrenic, and when I did shrooms, I became paranoid that I was becoming schizophrenic, but in fact, it was just fear fueling fear. Stop trying to claim that weed can cause mental illness as a fact, because it isn't, there isn't enough conclusive evidence due to the illegality of it. If it is made legal, then legitimate, unbiased studies can be performed and conclusive evidence might be able to surface. The same goes for the opposing argument. Let me repeat that so I don't have people jumping down my throat. THE SAME GOES FOR BOTH SIDES OF THE ARGUMENT. We can argue until we're blue in the face but that won't change the fact that neither of the sides know enough to form a legitimate conclusion.
Both sides will spew out the same "evidence" for their side.
It's very hard to get straight facts on illicit drugs, besides chemical make-ups and vague effects because of how different peoples minds are and the difficulty of testing these drugs. Also because it is illegal there is a strong HIGHLY biased force trying to give the drug a bad name. On the other hand, there are users who try to give it a good name.
/semi-rant like thing.