The fallacy of argumentum ex silentio occurs when an arguer presumes that because an author made no mention of an event, the author must have been unaware of it, or the event in fact never happened.

More specifically, the fallacy relies on these premises:

1. An author's writings made no mention of a particular event.
2. The event in question would have been important enough to the author's aims in writing, that the author would have mentioned it if the author had been aware of it.

From these premises it is concluded either that the author was ignorant of the event, or that the event is fictitious and in fact did not occur.

The problem is that there have been numerous cases recorded by historians where precisely this chain of reasoning went wrong, i.e, where it was later determined with certainty that an event did indeed occur (and was known by the author to have occurred) despite "silence" about it. Therefore, when employed as a deductive argument, it is an informal fallacy, with the second premise (above) being asserted hastily (and inaccurately). The reality is that authors can sometimes fail to mention things, even if those things would have been relevant to their project.