When I don't trust what someone says, then I need to say,

I don't believe you.

without 'in'.

But for the notion, it's a concept, so I think I need to say,

I don't believe in the notion.

but on Google, I could find a lot of results saying "believe the notion".

Is it possible to use both of them?

  • I'd go with your ideas rather than those in the results you mention. – Edwin Ashworth Jun 11 at 20:40
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    I would say that "believe in" is more like a life stance, a general viewpoint you take, but "believe" relates to some specific statement. Note the difference between "I don't believe God" and "I don't believe in God". – Weather Vane Jun 11 at 20:44
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    Certainly you can "believe a notion" if you choose. You can believe a proposition, a story, a person, a warning, an axiom, and so on. You don't have to believe in it, but you can do that too. Sometimes those things mean the same thing, sometimes they don't. – Robusto Jun 11 at 21:01
  • Although "believe in" is valid too (as @phoog explains), I suspect, a large percentage of the texts you found should be using the noun "belief" instead of the verb "believe". – Mikhail T. Jun 12 at 13:25
  • You believe a fact, you believe in an idea. – J... Jun 12 at 16:42

They mean different things. To believe something is generally to accept it as truth. To believe in is generally either to accept that something exists (believe in Santa Claus) or to align yourself with some ideology (believe in capitalism). People sometimes use the "ideology" sense with real people, however, leading to the nominally nonsensical I don't believe in Donald Trump, which seems to question his existence but actually just disavows his policies.

So, you probably believe the notion that there are cars that use petrol (gasoline) and diesel in their internal combustion engines, but you may nonetheless believe in the notion that they should be replaced with vehicles using alternative fuels.

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    "I don't believe in Donald Trump" is not nonsensical: it means something like "I don't trust Trump because I think he isn't reliable". See "believe in sb" entry in Cambridge English Dictionary. – Ruslan Jun 12 at 10:45
  • "believe ideology" and "believe in ideology" have quite different meanings. – Joshua Jun 12 at 17:18
  • @Ruslan Nominally, according to the Cambridge Dictionary: “in name or thought but not in fact, or not as things really are”. – wizzwizz4 Jun 12 at 21:38

The Merriam-Webster dictionary has a clear definition. It says:

Definition of believe in 1: to have faith or confidence in the existence of (something) Do you believe in ghosts? 2: to have trust in the goodness or value of (something) She believes in (the value of) regular exercise. I believe in working hard to achieve success. She doesn't believe in using pesticides. 3: to have trust in the goodness or ability of (someone) Despite his problems, his parents still believe in him.

That is clear, I think. Belief in is a kind faith or trust

If you look at definitions of believe (without the preposition), they all involve thinking that some statement or collection of statements is true, or that the person making the statement is telling the truth (ie that a statement that person is making is true.

There is one important exception to this. If you say "I believe in Marxism" or "I believe in Buddhism". Sometimes it means that the speaker believes that something exists: God, black holes, etc. Sometimes it means you trust or agree with something: democracy, President Trump or what or whoever.

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There are three common usages of "believe" that come to mind:

  1. I don't believe [statement]

  2. I don't believe in [entity]

  3. I don't believe [person or group of people]

Form 1. is pretty straightforward and is just a declaration of your opinion on whether or not the statement is accurate. Eg:

I don't believe pigs can fly


I believe blue is a good color

Form 2. is either (1) a declaration of your faith that something is capable of achieving its purpose or (2) that the thing exists at all. If you're talking about people, then saying

I don't believe in librarians

means that your opinion is that librarians are incapable of doing their job and if you went to a library you might try to find your books on your own. Saying

I don't believe in smartphones

means that you think you're better off without smartphones and maybe prefer to use a basic flip phone or no phone at all.


I don't believe in Bigfoot

means you don't think Bigfoot exists. Alternatively, if you're speaking among people who assume Bigfoot exists, you might instead be interpreted as saying you simply doubt Bigfoot is going to do well in life :)

For the third form,

  1. I don't believe [person or group of people]

I think this is just an idiomatic shorthand for

I don't believe the statements from [person or group of people]

If I said

I don't believe you

I'm just telling you that I doubt that whatever it is you last said is correct.

I think the third form could be an example of metonymy which wikipedia defines as

a figure of speech in which a thing or concept is referred to by the name of something closely associated with that thing or concept

In this case the "concept" being referred to is the statements of some group and the way you're referring to those statements is by the group itself.

This is just my opinion as a native English speaker.

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