You argue that a liberal drug policy would reduce the number of drug users, because criminals have no incentives to recruit new addicts. Your friend argues that such a policy would lead to a huge increase in drug users, because drugs are freely available. Who is right? Then you dig out the statistics of the Netherlands, a country with a very liberal drug policy, and show your friend that drug use has not increased. You proved him wrong. And claim a win on the debate. But then you re-examine the data and see that the drug use stayed relatively constant defying your argument, too. You were both wrong. Or wait maybe you were both right, because the two effects might balance out?
When two people present statements, there are four scenarios. A is right, B is wrong. A is wrong, and B is right. And both A and B are right, and both A and B are wrong! Often, we get confused, because assume that the two statements are exactly opposite, e.g policy reduces number, or policy does NOT reduce number. However, very often both people hold different opinions but not 100% opposite opinions. You argued that drug use would go down, and your friend that it would go up. In fact, you were both wrong, drug use stayed relatively stable!
Never think that you have won an argument by defeating someone else's arguments. Sure, you have proven them wrong, but your statement could well be wrong or right.