Sorry but that kind of thinking is worthless as you could used it for anything you do not care for not just marijuana smoking.
Coffee drinking, diet coke drinking, tap water drinking heavy exercise or no exercise or................
Yes, but if works in an argument about coffee, overexercising, etc, why does it not work in a case about marijuana? Just like we might not know that a friend of ours has a coffee addiction so severe that they cannot function normally at all unless they have at least 3 cups of coffee, but that the result definitely takes a toll on their body and mind... thus, the friend might be energetic to the point of abnormal exhaustion, or easily irritable or something, and we might think its only part of their personality.. when really its the overdose of coffee destroying their body and/or mind. Why is it not possible that it is the same with our marijuana-obsessed friends?
Maybe the fact that they are so calm and seem shy and quiet most of the time is NOT part of their personality but really they have somewhat "numbed" themselves each and every morning and prevent themselves from thinking clearly, let alone put together a decent sentence without needing to try extremely hard...? Maybe they are sleeping in class and cannot concentrate on one solid topic for more than 0.5 seconds, not because they are too energetic and "random" but because they are causing their body to work this by smoking a joint or two in the morning and between every class?
Of course, that comes with other disadvantages that their body and mind experiences, which we may not know of.
So how can we argue that constant marijuana-smokers are perfect fine when we CANNOT know?
Just like we cannot know what a caffeine addict might be going through.. or the stress an over-exerciser puts himself through..
We cannot feel what their bodies feel, and therefore, any argument of yours, BillRM, and those who also claim to know that "marijuana does not harm" is irrelevant.