Also Known As: Hindsight Fallacy
This fallacy, which is credited to David Hackett Fischer, occurs when it is assumed that people in the past viewed events with the same information or perspective as those analyzing these past events with the benefit of hindsight. The fallacy has the following form:
Premise 1: From the present perspective event A in time T is seen as X (a good idea, significant, a bad idea, etc.)
Conclusion: Therefore, event A was (or should have) been seen as X at time T.
The X above can include a wide range of evaluations, such as being a good idea, being of great significance, being a bad idea, being easily foreseeable, and so on.
This sort of reasoning is a fallacy because it is an error to infer that people in the past would (or should) see the events of their time from the perspective of those in their relative future. Obviously, the people in the past do not have the benefit of hindsight that those looking back possess.
It is not a fallacy to analyze past events from a present perspective, provided that the analysis is done in a way that attributes to those involved only the information they could reasonably be expected to have at the time.
For example, suppose that Sally marries Bill, and he seems fine until he becomes dangerously unstable. In this case, it would not be a fallacy to claim that it turned out to be a bad idea for Sally to marry Bill. It would be a fallacy to judge Sally as if she knew then what she only learned now. To use another example, if Sally did have adequate evidence that Bill was (or would become) dangerously unstable, then one would not commit this fallacy if one were to argue that she made a bad choice when she married him.
It also is not a fallacy to be critical of a person for what they reasonably should have known. For example, if Sally did not know about Bill being a psychopath because she married him a week after meeting him, it would be reasonable to argue that she made a poor choice in not getting to know more about him. This does not require having a perspective available only from the future and hence would not be fallacious.
Defense: The defense against this fallacy is to consider what a person in the past would have reasonably known at the time.
“It seems clear that Roosevelt must have known about the attack on Pearl Harbor and let it happen to ensure that we got into the war. After all, looking over all the historical data from the United States and Japan, the signs of an attack are so obvious. So, he surely must have known.”
Dan: “Did you hear? Kelly and Rob are getting divorced.”
Dan: “Well, Rob lost his job and…”
Lisa: “And she just dumped him as soon as she found out? Rob is such a great guy and I’m sure he’ll get a new job. I set them up, you know!”
Dan: “No. He didn’t tell her that he lost his job. He tried to find one, but he couldn’t and it kind of broke him. He started drinking and he wrecked the car while driving drunk.”
Lisa: “She should have known to never marry that loser!”