I don't think you two will see eye to eye on this, and can fill several more pages of the same banter.
What about cannabis in particular? Legalizing cannabis would be a good thing. Many studies show that it has medicinal value, especially for those that have cancer. Smoking anything is bad for you, but it doesn't always have to be smoked to be taken. As far as recreational use is concerned, I do not see it as any worse than alcohol or tobacco, and I actually personally think alcohol us much worse, yet is totally legal. For someone to do it in their own home should be legal. When you get in a car or operate other machinery, that's when you're crossing the line, and measures should be taken to deal with offenders that do so. As for industrial use, hemp is an incredible product and should definitely be legal. Industrial hemp plants do not have any near the levels of THC that could get someone high.
As for heroin, cocaine, lsd, etc, I see no need for it to be legal in any way.