The article that started this discussion does not address the likelihood factor at all. There is no data that you point to that supports this claim. All you state is that it is "common sense" - which is the "intuitive sense" that the article refers to, the "intuitive sense that the study actually cannot verify.
You are arguing that there is a correlation between marijuana usage and heroin usage. There is also a correlation between beer and heroin, between cigarettes and heroin, between addicted family members and heroin. Correlation is NOT causation, however. You could stop all people from smoking pot for the rest of time, and heroin use would still continue. There was no marijuana available in 19th century China, but opium addiction still flourished. Would abolishing rice wine, which most opium smokers drank at some point before they smoked, have ended the opium blight on society?
An example of correlation: eating fatty foods is correlated to colon cancer. Does this mean eating fatty foods causes colon cancer? No. Does this mean that the fat molecules prevent your white blood cells from attacking the cancer? No. Does this mean that the fat molecules counteract the anticarcinogenic action of the broccoli you also ate? No. We can make up any explanation we like, and we may even like one explanation more than another, for reasons totally unrelated to the evidence. But we do not know what the relationship between the two is, and cannot make a factual claim about it.
Now hearing about this correlation may change your opinion on your personal consumption of fried chicken and french fries. Fine. You don't have proof that fried chicken and french fries will give you colon cancer, but you make the choice not to take the chance.
Does this change in diet mean you will not get colon cancer? No. Does the correlation mean that fatty foods should be banned? No. Can you say anything factual at all about fatty foods and cancer? Just that they seem to be related, but, at this point, we have no scientific evidence what that relationship means.
But that is the difference between a claim of fact and an opinion, right? Opinion is based on personal belief. It may have no basis in fact at all. It may even cause a person to refuse to examine additional factual evidence because they feel their opinions are threatened.
Of course I can have an opinion without "absolute, incontrovertible fact". That is a straw man argument. I can have an opinion without knowing any facts at all! My opinion is that adultery destroys marriages. I have no statistics on the correlation between the two. I haven't defined my terms, such as "adultery" or "destruction." My opinion is based purely on my own personal beliefs.
But I would never make a factual claim that adultery destroys marriages without collecting some evidence, defining my terms, and examining some alternative explanations. And, no, I would never just utter a certainty, as you have done, on the basis of "common sense."
But what if fatty foods were already banned for some other reason? Wouldn't the correlation to colon cancer be yet another reason to keep fatty foods banned?
Isn't that what we're talking about with marijuana?
"But I would never make a factual claim that adultery destroys marriages without collecting some evidence, defining my terms, and examining some alternative explanations. And, no, I would never just utter a certainty, as you have done, on the basis of "common sense."
But if I approached you and asked you if we should vote nationwide to allow adultery and remove the social stigma associated with it, wouldn't "common sense" be enough for you to say no? Would you need a study before you could respond to that question?