When my ancestor died in Illinois 1903, she was survived by her husband who died in 1904. Her husband is not mentioned in her will and, more importantly, he is not listed in the court record listing all her heirs. In every other record I've seen during this time, the surviving spouse is included in the court order. His failure to be listed is "negative evidence" that he was not her husband when she died in 1903.
It's negative evidence because we are using the fact that something we expect to be a part of the document is "not there" (ie. his name is not on the heir list) as evidence of something. It is his absence that is the evidence.