Is named syllogism to a form of deductive logical reasoning, whose fixed structure consists of two different propositions acting as premises and a third as the conclusion of the reasoning. The first two are known as major premise or universal and minor premise or particular respectively.
The syllogism always takes place in the same three steps: It starts from a first premise or major premise, which supposes a general or universal statement, of a broad nature (1), then a second or minor one of a particular nature, specific with respect to a specific reality that we wish to contrast with the first premise ( 2) and thus finally obtain a conclusion (3).
For this reasoning to occur correctly, without incurring fallacies, the following rules must be taken into consideration:
- A syllogism must always operate based on the three terms already mentioned.
- The particular premise it cannot be the conclusion at the same time, nor be contained in it.
- The major premise must always be universal. From pure particular premises no true conclusion can be given.
- The conclusion cannot be more universal than the premises where it comes from.
- From negative premises no conclusion can be drawn.
- A negative conclusion cannot be obtained from affirmative premises.
- The premises must have common terms.
- The conclusion cannot deal with matters not contained in the premises.
If these rules are not observed, it is possible to incur fallacies or reasoning errors, that would suppose true a false conclusion or obtained by means of an erroneous deduction (even if it were true).
The most common of these errors can be:
- Semantic ambiguities. It occurs when the terms of the premises can be interpreted differently, and the explicit meaning from which the deduction will start is not clear.
- Paralogisms. Errors in the deductive chain, usually unnoticed by its speaker, since they formally appear to be valid, but are not.
- Sophisms. Intentional false reasoning, with manipulative purposes, which is disguised as valid reasoning when it is not.
- Subject ignorance. When you start from weak premises or that contain errors or information gaps, you often incur fallacies that affirm the impossible as true.
- Generalizations. It occurs when a particular conclusion is extended to a group, ignoring what is stipulated in the premises.
Examples of syllogisms
- a) All men are free
b) Nicolás is a man
c) Nicolás is free
- a) Some birds are flying
b) I like flying animals
c) I like some birds
- a) No man can fly
b) Canaries fly
c) Canaries are not men
- a) The planets are all round
b) Jupiter is a planet
c) Jupiter is round
- a) Sports cars are expensive
b) The new Ferrari is a sports car
c) The new Ferrari is expensive
- a) No fish is mammal
b) Whales are mammals
c) Whales are not fish
- a) Some scorpions are poisonous
b) Poisons can kill us
c) Some scorpions can kill us
- a) All violence is deplorable
b) Some protests are violent
c) Some protests are deplorable
- a) Whenever I eat fruits I get dizzy
b) Whenever I feel dizzy, I vomit
c) Whenever I eat fruit, I vomit
- a) No person breathes into a liquid
b) Water is a liquid
c) No person breathes in the water
- a) Some soldiers are psychopaths
b) Some Swedes are military
c) Some Swedes are psychopaths
- a) Colored butterflies are eye-catching
b) Showy animals die first
c) Colored butterflies die first
- a) Some asteroids are made of iron
b) Iron is a magnetic mineral
c) Some asteroids are magnetic
- a) Every day it rains
b) When it rains, I take out the umbrella
c) Every day I take out my umbrella
- a) My grandfather looks like Clint Eastwood
b) Clint Eastwood has a horse face
c) My grandfather has a horse face
- a) All flowers have perfume
b) All roses are flowers
c) All roses have perfume
- a) No female is male
b) All males are born with a penis
c) No female is born with a penis
- a) No politician is poor
b) The poor generally earn little money
c) Generally no politician earns little money
- a) No thief is honest
b) Some people are honest
c) Some people are not thieves
- a) Every winter is cold
b) When it’s cold I get sick
c) Every winter I get sick
Examples of fallacies
- a) No man breathes underwater
b) Divers breathe underwater
c) No diver is a man
- a) All dogs eat
b) All men eat
c) All men are dogs
- a) Aristotle was a philosopher
b) Aristotle was Greek
c) Every philosopher is Greek
- a) All men breathe
b) No woman is a man
c) No woman breathes
- a) Cows are mammals
b) Bats are mammals
c) Cows are bats
- a) No pig can fly
b) The dining room table cannot fly
c) The dining room table is a pig
- a) Every Monday I’m late for work
b) If I’m late for work I get fired
c) I no longer go to work on Mondays
- a) Some immigrants are criminals
b) We have a lot of crime in the city
c) Crime is due to immigrants
- a) Some reptiles have legs
b) All dogs have legs
c) Some reptiles are dogs
- a) My grandmother passed away the day before
b) Broccoli went up in price
c) The broccoli rises because my grandmother dies