How to respond to “philosophical” questions that aim to undermine a position or derail a discussion? A colleague of mine enjoys using philosophical questions to undermine ideas that are different to his own and to derail discussions that are not in his favour. At first I thought he was offering genuine perspectives, but overtime I realised he was just being defensive and finding enjoyment out of disrupting discussions. 

To give you some examples, here are some of his frequent techniques: 


The you-can’t-know-for-sure: 


  The evidence may seem to be true but it could be wrong, and we can never know that for sure. You can only believe in evidence. Therefore, making claims about anything, with or without evidence, is meaningless. 
   
  The human senses and the human mind are limited and fallible, and to say that an idea is more correct than another is just your opinion based on a fallible experience.

Disagrees with the definition, or shifts to a different definition, or question the meaningfulness of definitions and language in general:


  I define X as something different to how you defined X, and according to what I think of X, I am right. You can think of X how you want but it doesn’t matter to me.
   
  
  “X” only means X because humans defined “X” to be X. But “X” is just a subjective idea that doesn’t exist in reality. Therefore, anything you say about X, your ideas are on based on something non-objective and arguable.

All things are just perspectives:


  To say that I’m inconsistent with my argument or to say that I’m wrong is just a perspective. There are many perspectives and just because from one perspective I’m a certain way, doesn’t mean I really am.



I usually enjoy genuine philosophical perspectives in all discussions, but this colleague never offers his philosophical ideas at the beginning of talks and never mentions them when we agree with him, but only throws them out selectively against ideas during the middle of discussions. The worst is that he speaks them with a triumphant and condescending tone of voice over everyone else as if he is so clever. 

Question: When he uses these types of philosophical questions as a way to undermine someone’s idea or to derail a discussion, what is an appropriate way to interfere with his remark in a way that doesn’t confront him, protects the other person from appearing to have being undermined or devalued, and restores the direction discussion to what it was before?
Is this colleague really an academic? How on earth did he get a job in academia? How does he manage to retain it? He sounds like a teenage boy from your description of his style of arguing.

He's a troll, so best ignored. He also sounds like he's realised he's way out of his depth, and this is bluster to cover it up.

Formulate a single sentence that lets him know he's been heard, but that it was irrelevant, and that you've already moved on. Use it. Vary it, but keep it short, and always end with an invitation to the interrupted speaker to carry on where they left off. The troll should get the message that he's been sidelined, and troll elsewhere.

"I don't think we need to surrender to nihilism just yet, do we? [turn back to the interrupted speaker] Now, as you were saying ..."

Then just exclude him from discussions in the future. Take your fellow discussants to another room, away from the troll, if you have to.