It's an attack on the poster's knowledge, and stating that they do not have the information to discuss it. Whether or not that's valid is a different argument altogether. But again, I was not stating I agree with headhunter vs. rat, I was not stating that headhunter was not using an ad hominem argument. I was only saying, and this is the third time I'm clarifying this, questioning knowledge is not an attack on character. It may be an attack on the poster in terms of attack on their knowledge level (if you want to use a really lose version of the word 'attack'), but no judgments at all were stated about his character.