[torben-friis]

Admittance of ignorance is undoubtedly better than a lie, in general.

[ars]

I wrote "I can't know", not "I don't know".

This is a question about the limits on knowledge, not about someone's personal knowledge.

[mensetmanusman]

It’s impossible to say whether it is a lie, because of incompleteness.

[NateEag]

Let's try the more charitable "unjustified certitude" instead of "lie".