This fallacy occurs when it is concluded that another person has a certain mental quality because the person drawing that conclusion has that quality. This fallacy has the following form:
Premise 1: Person A has mental quality (or qualities) Q (a belief, a skill, knowledge, or tendency to act a certain way, etc.).
Conclusion: Person A concludes that person B has Q.
The error being made is specific type of False Analogy: person A is drawing a conclusion about person B based on the unsupported assumption that A and B are alike. Without adequate reason to think A and B are alike enough in relevant ways, concluding that they are alike regarding the quality in question is unjustified.
This fallacy is often fueled by the false consensus effect. This is a cognitive bias that inclines a person to think that their attitudes and beliefs are also held by the general population.
This fallacy can be avoided by making an adequate argument from analogy. This would involve providing the key premises establishing that A and B are alike in ways relevant to the quality in question.
The fallacy was named by William James. He noted that psychologists are particularly prone to ascribing their own standpoints to those they examine. But a person does not need to be a professional psychologist to commit this fallacy.
Getting a bit philosophical, one classic problem in epistemology is the problem of other minds. In my own case, the problem is determining how I would know that other beings have (or lack) minds like my own. More practically, the problem is determining if a person’s words and actions match what they believe and feel.
Philosophers have generally tried to solve this problem using an analogical argument. The usual idea is that I would infer that because I have mental states (thoughts and feelings) and other people are like me, they also (probably) have mental states. Critics of this approach point out that it is a weak argument by analogy and that extending it to all people would be a Hasty Generalization because the sample size must be one person. This person would be me in my case, you in your case.
If the problem of other minds is taken seriously, then making inferences about the mental qualities of other people would seem to always be ill founded.
Defense: The defense against this fallacy is to consider whether there is adequate evidence to infer that someone else has the same mental qualities (beliefs, interests, values, etc.) as you. From a practical standpoint, it is best not to get bogged down in the problem of other minds.
Christine: “Thanks for coming to dinner! I made bacon burgers. With cheese!”
Christine: “I really like them. I figured you would, too.”
Florence: “I’m a vegetarian. Do you have anything I can eat?”
Christine: “Well, you can put the cheese, lettuce and onions on the bun.”
Florence: “I don’t like onions. Or lettuce.”
“I’m sure those people will help me push my car out of the ditch. After all, I’d help someone who is in the same predicament.”
Bob: “Did you hear that the legislature just voted on a law legalizing same sex marriage?”
Gretchen: “No way!”
Bob: “Really. It is going to the governor.”
Gretchen: “There is no way she’ll sign it!”
Bob: “Really? Why?”
Gretchen: “Well, I wouldn’t! So, I’m sure she won’t!”
Bob: ‘Uh, huh. Well, would you have voted for the law if you were in the house or state senate?”
Gretchen: “Hell no!”
Bob: “And yet the bill passed…”
Bill: “I’m sure that no one would like that movie.”
Bill: “Well, I did not like it.”