The fallacy of uncritical presentism (aka nunc pro tunc or "now for then") occurs when an arguer assumes that prevailing contemporary views can be used to interpret and evaluate past events, disregarding the historical difference in the surrounding culture and values.

Here is an example:

"Columbus must have been an idiot, because he thought, upon reaching North America, that he had gone all around the world and reached India. He was off by many thousands of miles, and didn't even know what continent he was looking at. What a dope!"

The problem here is that, regardless of whatever else Columbus might have been, he was not stupid; his knowledge of the geography of the world was ahead of the vast majority of his contemporaries, and he reasoned with the information that he had available at the time. That he made a very large geographical error was not due to a lack of intelligence, but rather due to a lack of knowledge on the part of the entire European civilization in the late 15th century.

Note: Presentism is the opposite of historicism, which is constructing a description of history in its original context apart from a contemporary perspective. When presentism is tempered with historicism, it is not necessarily a fallacy. But when presentism ignores or dismisses the differences in perspectives from an earlier time period in order to judge persons or events as they would be evaluated today, then it is an uncritical presentism, and is fallacious. It is invalid because it presumes that people in a previous time period either did have, could have had, or should have had the same perspective or worldview as people in modern or contemporary times have.