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I often come across a certain fallacy when talking to people regarding agreeing with someone about something necessarily means I agree with everything else. Here is one example:

  1. John is an atheist, therefore he believes that green is a nice color
  2. John believes that 2 + 2 = 4
  3. I also believe that 2 + 2 = 4
  4. Therefore, I agree with John
  5. As such I also agree that green is a nice color

I agree that the fallacious point here is that the statement number 4 is only valid to the scope of 2 and 3, not generically. However it seems that I come across this fallacy way more often than I would expect, so I guess that this must a common fallacy. One situation that I came across was the following one:

  1. Vegans believe that they shouldn't eat or drink animal sourced products
  2. Vegans believe that they should substitute dairy milk with plant milk
  3. I believe that I should substitute dairy milk with plant milk (because I don't like dairy milk only)
  4. Therefore I agree with vegans
  5. Therefore I agree I shouldn't eat or drink animal sourced products (not true from here)
  6. It follows that I'm a vegan

This was told me by a person who saw me drinking plant based milk and he immediately came to the conclusion that I was vegan. Another one was someone who believed I had religious restrictions to alcohol since I don't drink alcohol (I just don't like to drink alcohol). Also sometimes when I agree with a statement from a politician I'm often labelled a supporter of that politician and agree with everything else this politician's party promotes.

Does anyone know what is this fallacy, if it is named and there are other resources about it? This fallacy is often used against me to label incorrect things about me, so I want to know how to prevent people from doing that to me and to be more precise with my points and avoid misunderstandings.

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Regarding your 2nd example:

  1. Vegans believe that they shouldn't eat or drink animal sourced products
  2. Vegans believe that they should substitute dairy milk with plant milk
  3. I believe that I should substitute dairy milk with plant milk (because I don't like dairy milk only)
  4. Therefore I agree with vegans
  5. Therefore I agree I shouldn't eat or drink animal sourced products (not true from here)
  6. It follows that I'm a vegan

The actual falsity started from step 4 (not your step 5). You just agree with a subset of a vegan's qualifications, not all of those qualifications which as a whole defines who can be called a vegan. Basically you're not qualified for all conditions as a vegan, yet a concept has been replaced subtly. But the root exact logic fallacy of your question is very hidden and is called Fallacy of the undistributed middle in logic jargon which is a type of Formal fallacy. Below is the gist from the linked source:

The fallacy of the undistributed middle takes the following form:

  1. All students carry backpacks.
  2. My grandfather carries a backpack.
  3. Therefore, my grandfather is a student.

Now hopefully you can spot the fallacy easily since backpack is the undistributed middle of a syllogism here, thus the formal deductive logic (the concluding statement 4 above) is invalid.

Btw, this looks extremely similar to Hasty generalization which is actually a type of inductive Informal fallacy. Hasty generalization draws a universal conclusion based on a small number of particular samples.

For example, "I’ve met two people in Nicaragua so far, and they were both nice to me. So, all people I will meet in Nicaragua will be nice to me"...

As the old saying goes "The devil is in the details", real life is full of these partially correct fallacies. And sophists are quick to use such confusion to replace with a different concept for their advantages...

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  • Thanks for the answer, that's what I was looking for. Yeah, I find people who do that obnoxious to talk to and the person who did that to me was obnoxious in several ways, not only this one. I wish I could learn how to discourage people from doing that to me now. – Gabriel Diego Apr 4 at 11:16
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It's an example of hasty generalization; they are generalizing from one trait about you, to other traits that you don't have.

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  • Hasty generalization is when a small sample of the population is used to generalize the whole. Like saying "I know a vegan who is nuts, therefore all vegans are nuts". What I'm describing is more the other way around, it is inserting me into a group due to having just one caracteristic of it. – Gabriel Diego Apr 4 at 11:13
  • @GabrielDiego they're hastily generalizing, then applying the hasty generalization to your specific case. "Vegans substitute plant milk for dairy milk. Therefore, anyone I see substituting plant milk for dairy milk must be a vegan." Of which you are a particular case. – causative Apr 4 at 18:04
  • @GabrielDiego It could also be called the en.wikipedia.org/wiki/Package-deal_fallacy – causative Apr 4 at 18:06

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