An event happening differently for different possible states of whatever you want to know about
("What is evidence?", Eliezer Yudkowsky, <a href="https://www.lesswrong.com/posts/6s3xABaXKPdFwA3FS/what-is-evidence">https://www.lesswrong.com/posts/6s3xABaXKPdFwA3FS/what-is-evidence</a>)