Modus Tollens / Popperian Syllogism for the Black Swan


If it is a swan, then it will always be white.

It seems like Popper is not asking so much people to build syllogisms, but rather DEPENDING on Modus Tollens as an engine for falsification.

So that, if I propose -

  • If it is a swan, then it will always be white.

And then go ahead and execute my testing model by traveling to Australia.

Now I have:

If it is a swan, then it will always be white.

I just saw a black swan.

  • If A then B
  • Not B
  • Therefore, Not A

But, here is my confusion: it seems like my hypothesis has some structural issue for Modus Tollens to take effect.

Using what I proposed I get:

  • If it is a swan, then it will always be white.
  • I just saw a black swan. [Not B]
  • It is not a swan. [Not A]

The "it" here suffers dually - from being ambiguous, as you need to stop for a second and figure out that the "it" here refers to the Antecedent "it" in the Conditional statement.

But, WORSE! - if I were truly going to go with Not A, saying

"It is not a swan." [Not A]

makes no sense.

So, my question is: how should I structure the white swan hypothesis Conditional Statement so that the "Not A" side of the syllogism under the process of Modus Tollens looks a bit more trenchant?


Posted 2019-01-27T03:23:40.170

Reputation: 69

Despite much thought that has gone into this question, I still am not clear on what the problem might be. How about “I just saw a non-white swan”? Would that make the terminology more precise? – Mark Andrews – 2019-01-27T05:02:07.173

Hi Mark. Seems like what I’m asking may be a more Linguistic question. For example in: If George has a dog, then he is a pet owner. George is not a pet owner. Therefore, George does not have a dog. Here there is a striking correlation between the parts. The Conditional, Denying the Consequent and the Conclusion. It really RINGS out when you read it. How could we write a Black Swan syllogism to create the same effect? – oaktrees – 2019-01-27T11:29:38.500

What I'm trying to figure out is a is a way to rewrite the Conditional so that I can arrive at a 3-part syllogism - 1[If A, Then B], 2 [Not B], 3 [Therefore, Not A] - that ends in something like "Therefore, not all swans are white." – oaktrees – 2019-01-27T14:04:13.393

black swans, yay! – None – 2019-02-27T09:08:04.077



It is quite simple.

The "general law" we have assumed as an hypothesis is :

"Every swan is white"

that, according to the language of predicate logic, is :

(1) "for every x, if x is a Swan, then x is White" [in symbols : ∀x(S(x) → W(x))].

Yoy are travelling in Australia and you find a black swan, that is :

(2) "there is an x such that, x is a Swan and x not is White" [in symbols : ∃x(S(x) ∧ ¬W(x))] .

Call that Swan s; from (2), by Existential instantiation we have :

S(s) ∧ ¬ W(s).

Form (1), by Universal instantiation we have :

S(s) → W(s).

But the two formula are contradictory, because P ∧ ¬Q is the negation of P → Q, and thus we have to conclude with the negation of the initial hypothesis, i.e. we have falsified the purported "general law" :

"Every swan is white".

The argument above can be expressed also with Modus Tollens.

We have S(s) → W(s) and from S(s) ∧ ¬ W(s), by Simplification we get : S(s) and ¬ W(s).

Now we have : S(s) → W(s) and ¬ W(s) and by MT we conclude with :

¬ S(s).

This contradicts S(s) and we are done.

It is not exactly a Syllogism because a syllogism needs three terms, like e.g.: S,P and M.

In our example, we have only two : Swan and White.

Thus the inference is :

1st premise : "All Swans are White" [A-type : Universal Affirmative : "All S are W"]

2nd premise : "Some Swans are not White" [O-type : Particular Negative : "Some S are not W"]

With them, we may have a valid Baroco syllogism, concluding in O-type : Particular Negative.

In fact, the conclusion we have reached denying the first premise :

"Not every Swan is White" ["Not all S are W"]

is a Particular Negative, because it amounts to "Some Swans are not White" ["Some S are not W"].


Posted 2019-01-27T03:23:40.170

Reputation: 33 575

1Hi! When I read your reply I felt impressed. I think you're right. You've illustrated the mechanics of Modus Tollens clearly. Seems like I may be asking a question more related to the linguistic structure of the syllogism. For example: If George plays tennis he will wear sunscreen. He is not wearing sunscreen. He will not play tennis. These's such a tight linguistic correlation there between the elements of the Conditional, Denying the Consequent, to the Conclusion. Rings out just like a bell. How could we write the Black Swan syllogism to get the same effect? – oaktrees – 2019-01-27T11:05:26.793

I was about to answer, but your answer is already in-depth and to the point, bravo +1 – SmootQ – 2019-02-27T09:34:12.867

1@SmootQ - you are welcome :-) – Mauro ALLEGRANZA – 2019-02-27T09:41:56.740