@hue-man,
hue-man;131923 wrote:Can we ever be positively (not absolutely) certain that any particular thing doesn't exist?
Yes. We can know that there are no round squares.
hue-man;131930 wrote:I should have been more specific. Can we know that an invisible unicorn doesn't exist?
Yes. We know this for a variety of reasons. First of all, things that are invisible are still detectable. We can detect air that is invisible to us. Second, an invisible unicorn would be problematic from the standpoint of the possible materials that it could be, and from the fact that eyes require non-invisible parts in order to function.
Now, if you go further, and start to ask about something that is not detectable in any way, then I will respond with saying that you are really and literally talking about nothing.
hue-man;131930 wrote:I changed it from know to positively certain after posting. By positively certain I mean can we verify that something doesn't exist by means of empirical study or logical decidability. Can we know that unicorns don't exist on a another planet in a galaxy far far away?.
...
If by "unicorn", you mean a horselike animal with a single horn, then we do not know if any such things exist on a remote planet. But we do know that they do not exist here. By "we", of course, I do not mean absolutely every person, as some people are stupid, crazy, ignorant, and/or other things that may prevent them from knowing such things. There are people with silly beliefs who ought to know better, but they do not. But as far as what life, if any, exists on remote planets, we are not in a position to say much about it, unless the proposed thing is impossible.