Which fallacy is assuming to know today a belief of someone from the past?

A common form of this is the statement:

If Thomas Jefferson were alive today, he would be an atheist.

Or, If Jesus were alive today he would be the presidential candidate for the Green Party and Bernie Sanders would be his running mate.

I haven't been able to pin down the name of this type of fallacy which claims to know the opinions of people when they cannot be known.

Any suggestions?

Thanks,

Dean

I would say that this simply is an assertion rather than an fallacious argument. However, this assertion can be a result of the historian's fallacy (see https://www.logicallyfallacious.com/tools/lp/Bo/LogicalFallacies/103/Historian-s-Fallacy<>).

it's not clear to me that this is an actual fallacy. It's just a statement that you can ask for justifications for, and when such justifications are given, then there will be potential for fallacious reasoning. Only after supporting reasons are given for why Thomas Jefferson would be an atheist if he were alive today can there be an argument susceptible to erroneous thinking.

historian's fallacy

Thanks to all of you! I'm still learning the interface and I guess my previous replies were anonymous and private.

Thanks for helping me to clarify the difference between an unsupported assertion and a fallacy.

It seemed to me to be a fallacy to claim to know what Jefferson would have believed. So I appreciate the comments and will think more about it. It is helping me refine my understanding of the categories and terms.

Hey Dean, thanks for the question:

The question as phrased: "Which fallacy is assuming to know today a belief of someone from the past?" is not fallacious in the least". In many cases we can easily surmise a historic person's beliefs by their writings or attributions without assuming anything. If we impose our own modern beliefs on them we are then transferring to them a conclusion that is not necessarily their own.

While the Jesus reference is too much of a speculative non sequitur to be taken seriously- I think it's safe to dismiss it as nonsense. The Thomas Jefferson reference however sounds to me like a variation of the Transfer Fallacy where the modern claimants own atheistic ideas are attributed to a famous person to justify their own beliefs. Jefferson's deist beliefs are well-documented and recorded. There is no need to assume anything. Likewise, to impose an atheistic conclusion to his voluminous records and writings is much more of a cognitive bias than an error in reasoning.

Thanks for the further comments. It looks like the Jefferson statement potentially contains several fallacies.

It also brought to light several other issues I had not thought of, like cognitive bias vs. logical fallacies.

Dean