How do you know?

Let’s say hypothetically you are engaging in an argument with someone, and you provide all manner of facts to support your claims. These facts are provided by scientific journals, historical texts, dictionaries, everything you can manage to prove your point is right. But then the person just keeps asking over and over again, “how do you know that’s true.” Over and over again, putting the onus on you to keep having to prove you are right. How do you counter that person? 

For any belief, intellectually honest people must ask themselves this question: What would it take for me to change my mind? 

The situation you face is, as mentioned by others, the “Effectively Inefficient Inquiry Infinitum.” You will only get past that if you change the game. You must ask, “What would it take to change your mind?” If the response is there is nothing that would change my mind, then the discussion is over. If your discussion partner lists what it would take to change his mind, then you have something to work with. 

The second item to remember is the typical progression of the discussion (in one form or another): “But you can’t be 100% sure of anything.”

I think it is essential to note that it is true we can never be absolutely certain of anything other than metaphysical axioms, so we must assign a value to any proposition based upon the available evidence. To dismiss a proposition on the basis it hasn’t been proven beyond all possible doubt is fallacious reasoning if one seeks knowledge. We can, however, attain an epistemological certainty, which, loosely stated, means beyond a reasonable doubt, while keeping our minds open to additional evidence or a different interpretation of available evidence.

Always obtain an agreement on the basic terms of the discussion. Knowledge, for example, is the correct identification of the facts of what exists, of reality. This is the most straightforward definition that I have encountered. Yet, it is an important concept to keep in mind. The purpose of the scientific method is to correctly identify the facts of reality, but such is beyond this discussion.

In your case, do not confront the person with facts and logic and all that stuff. Instead, do the Boghossian approach. 

In my experience, people never change their beliefs by being punched in the head with facts. Most people believe what they believe because they want to believe what they currently believe. Facts are not important. This was first discovered in 1954 by Leon Festinger in his book “When Prophecy Fails.” Michael Shermer made an addition to Cognitive Dissonance Theory in his book, “Why People Believe Weird Things.” So, if your goal is to change another person’s belief, you must use a different approach other than facts.

Peter Boghossian suggested a strategy to change a person’s belief. To be successful, he said, the person whose belief you want to change must reconsider how he arrived at the belief under discussion. If your goal is to change his mind, as distinct from pontificating (which is better done in front of a mirror), then you need to get him thinking about how he arrived at the belief.  

Boghossian’s book, “How to Have Impossible Conversations,” is an excellent manual on how to do this.

 I’ve come to a different conclusion, and I’m having difficulty understanding where you’re coming from. I assume you must know some things about this that I don’t. Could you tell me more about where you’re coming from so I can understand better? 

The more ignorance you admit, the more readily your partner in the conversation will step in with an explanation to help you understand. And the more they attempt to explain, the more likely they are to realize the limits of their knowledge and epistemological errors made along the way.

If you ask someone a direct question and he obfuscates or refuses to answer, ask him to ask you the same question, and you answer it. Other Boghossian suggestions:

 “That’s an interesting perspective. What leads you to conclude that?”

 Say, “I’m skeptical,” not “I disagree.”

Now, you have put the bouncing ball into your discussion partner’s court, and you no longer need to be defensive or provide more facts he will ignore.

 

This would be an example of argument by repetition , as they are simply repeating "How do you know" rather than actually presenting any new points.

 

Could also be seen as shifting of the burden of proof as they're continuously trying to make you prove that your evidence is valid, rather than trying to invalidate it themselves. The validity of any piece of evidence can be challenged, as scientific studies have yielded bad or fraudulent results, historical records have been inaccurate or forged, etc. But, a person arguing in good faith would bring some reason to probe those issues specifically.

From a practical standpoint, if someone does this and you continue to humor them, then there will inevitably be a point where you're relying on some piece of information that you can't personally verify. Maybe you were standing in the lab watching this particular study being performed, but how do you know the scientists weren't just actors? Eventually there comes a point where you have to acknowledge that you can't convince this person that you are correct, and you can stop arguing at any time. I generally end by pointing out that the possibility of all of the evidence in support of my point being faked is highly unlikely.

Ah, the age-old tactic of the repeat offender. They're pulling out the "Effectively Inefficient Inquiry Infinitum", a classic and frustrating move. It's like dealing with a parrot with an insatiable appetite for interrogation. They just keep squawking "How do you know", rendering every valid source, fact and argument you present as merely marshmallows being launched into the black hole of their doubt. Quite exhausting, isn't it?

To counter, you could opt to take the high road and patiently provide your sources and explain the logic, understanding, however, that you might as well be providing a gourmet meal to someone who insists on eating nothing but cheese doodles. You simply won't get through as they have transformed themselves into the vortex of so-called Socratic questioning, potentially with no real intention to understand at all.

Alternatively, you could surrender yourself to the universal laws of comedy. Charity donating, soup kitchen working, life-saving bananas have a peel for a reason, after all. Slapstick can save the day. So, gently slip in a zinger, "Well, my dear Inquisitor General, it's similar to how we ALL know that wearing socks with sandals is a fashion crime. Some things just ARE, you see." Or perhaps, "Your never-ending skepticism reminds me of my five-year-old’s insistence that the tooth fairy and Santa are having Thursday brunches with Bigfoot. Sometimes, we just need to believe in reason, logic and the power of good references."

Remember, facts are stubborn things, but humor can be even more so. Keep your wit about you.