From the internet I saw an example of an Affirming the Consequent fallacy :

If it's raining then the streets are wet.
The streets are wet.
Therefore, it's raining

I'm trying to make two examples.

1. If the animals are herbivore then they will eat plants
2. My dog eat grass
3. Therefore my dog is herbivore

I wonder, should I put "therefore dogs are herbivore" in number-3 in order it's a valid Affirming the consequent fallacy ?

If human beings are sinners then they will do sin
My toddler lied
Therefore my toddler is a sinner

The question :
Are the two examples above Affirming the consequent fallacy ?

I wonder, in example-1, should I put "therefore dogs are herbivore" in number-3 in order it's a valid Affirming the consequent fallacy ?

I wonder, in example-2, should I put "therefore toddlers are sinners" in number-3 in order it's a valid Affirming the consequent fallacy ?

Thank you.

  • 1
    Both of your examples move from sentential logic to predicate logic (i.e. your second premises involves "my dog" and "toddlers lie" respectively but the example doesn't require any such shift). Can you remedy this in your examples?
    – virmaior
    Jul 13, 2018 at 4:37
  • 1
    See Affirming the consequent for definition and examples. Jul 13, 2018 at 6:41
  • 2
    ALL examples must have the logical form : "if P, then Q; and Q. Therefore P". You van have an infinity of them, using any pair P,Q. Jul 13, 2018 at 11:09
  • 2
    "If Napoleon was the Emperor of Russia, then Obama was the US President. Obama was the US President. Therefore Napoleon was the Emperor of Russia." Jul 13, 2018 at 11:14
  • 2
    Formally, they are not correct examples, as per 1st comment above. The "logical form" must be "if P, then Q, and Q. Therefore P". In your first example, we have : "1. If the animals are herbivore, then they will eat plants"; if we read it as "if P, then Q", we have that Q is "animals will eat plants". Thus with "2. My dog eat grass" the issue is that it is not a new "instance" of Q, but it is a different sentence. Full stop. Jul 13, 2018 at 12:19

2 Answers 2


You're examples are not per se wrong ... but you're overcomplicating things for just giving an example of "affirming the consequent."

Look at Mauro's example:

(1) P -> Q (2) Q Therefore P

That's sentential. The key is that P always has to mean the same thing such as P = emperor of Russia and Q = earth orbits the sun (any P and Q really).

What you're doing is more complicated:

  1. Ha -> Eap
  2. Edg
  3. Therefore Hd

You're using what we call predicate logic like examples. (a = animal; p = plant; g = grass; d = dog E = eat. H = herbivore).

Ha = Animal is a herbivore Eap = animal eats plant Edg = Dog eats grass Hd = Dog is herbivore

There's a lot of moving pieces that complicate things (and are hidden). You really need to add (for completeness):

g = p (grass is a plant) [I'm cheating on the symbolization here] d = a (dog is an animal)

Or to put it another way, the same fallacy in predicate logic is:

  1. Ha -> Eap
  2. Eap
  3. Ha

which could then be stated sententially:

  1. If an animal is a herbivore (H) --> then the animals eats plants (P)
  2. an animal eats plants (P)
  3. Therefore the animal is a herbivore (H)

and you're slipping up terms. Is the fallacy still there? YES. But there's also some definitional bits missing that just make it more confusing.

  • I think I understand it now, virmaior. Thank you for your explanation. I have another question (if you don't mind), suppose someone say something like this "that dog eat grass, therefore that dog is a herbivore" is the sentence fallacy ?
    – karma
    Jul 13, 2018 at 19:54
  • 1
    a single sentence is not a fallacy, because fallacies only apply in arguments...
    – virmaior
    Jul 14, 2018 at 0:04
  • Oke... thank you virmaior. I'm sorry... one more question, when someone else ask "how come you say like that ?" and he answer : "because an animal which eat grass is a herbivore", is his answer a fallacy ?
    – karma
    Jul 14, 2018 at 3:12
  • 1
    I would say NO. because again, the term "fallacy" applies to an error in the reasoning of an argument in philosophy. It might mean something else somewhere else. What you might fairly call it is a "mistaken inference" -- because the inference that animal that eats grass must be a herbivore but the mistake is that an animal that eats only grass would be a herbivore but that it eats grass does not exclude it from eating other things.
    – virmaior
    Jul 14, 2018 at 13:43
  • It turn out logic is very difficult to me, LOL. Thank you very much for your kind respond and explanation, virmaior.
    – karma
    Jul 14, 2018 at 16:21

The previous answer and comments are adequate to address the main question. I would just like to add for the benefit of the last part, the "I wonder..." part:

The question here seems to be whether you need to use a universal (all dogs) as opposed to an existential (my dog) Quantifier. Those quantifiers are part of Predicate language while, to evaluate for this fallacy, Sentential language will suffice. Of course you can still use Predicate, you will simply be working with more information than strictly needed to detect the fallacy. When you use "my dog" it is implied that your dog is a member of "all dogs" and if you use "all dogs" then your dog is automatically included. But the two sentences are structurally equivalent.

In short both sentences are equally valid as regards being an Affirming the Consequent fallacy.

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