Difference: Syllogism vs Logical connectives
Standard format: logical connectives
Logical connective: if then
Logical connective: Only IF
Logical Connective: UNLESS
Logical connective: otherwise
Logical connective: When, Whenever, every time
Logical Connective: Either OR
Demo Q: Only if: bored TV brother (CSAT 2012)
Demo Q (If, then) Professor Headaches (CAT’98)
Demo Q: Either or: derailed/late train (CAT’97)

Difference: Syllogism vs Logical connectives

Syllogism (all cats are dog) is a common and routinely appearing topic in most of the aptitude exams (Bank PO, LIC, SSC etc). But Logical connectives is rare. However, in UPSC CSAT 2012 the topic was asked, therefore, you’ve to prepare it.

Syllogism	Logical connectives
Contains words like “all, none, some” etc. Can be classified into UP, UN,PP and PN. Already explained in previous articles.	Contains words like “if, unless, only if, whenever” etc. can be classified into 1, ~1, 2, ~2 (we’ll see in this article)
Have to mugup more formulas, takes more time than logical connective questions.	Less formulas and quicker than syllogism.
Question Statements: All cats are dogs some pigs are cats no dogs are bird Conclusion choices: Some cats are dogs No birds are cats some pigs are birds Some pigs are not birds	Question statements: I watch TV only if I am bored I am never bored when I have my brother’s company. Whenever I go to the theatre I take my brother along. Conclusion choices: If I am bored I watch TV If I am bored, I seek my brother’s company. If I am not with my brother, than i’ll watch TV. If I am not bored I do not watch TV.

Standard format: logical connectives

If, unless, only if, whenever, every time etc. are examples of Logical connectives.
Whenever you’re given a question statement, first rule is: question statement must be in the standard format.
The standard format is
****some logical connective word *** simple statement#1, simple statement #2.
It means, the question statement must start with a logical connective word, otherwise exchange position. For example

Given question statement	Exchange position?
If you’re in the army, you’ve to wear uniform	no need because the simple statement containing “IF” is given in the beginning. This is already in the standard format.
You’ve to wear uniform, if you’re in the army	We need to exchange position because the part containing “IF” is not given in the beginning of this statement, given statement is not in standard format. Therefore, Rewrite given statement as If you’re in the army, you’ve to wear uniform.
You’ve to salute, whenever Commanding Officer comes in your cabin.	Need to exchange position. Because statement doesn’t start with the logical connective “whenever”. Therefore rewrite the given statement as Whenever CO comes in your cabin, you have to salute.

Now let’s derive valid inferences for various logical connectives.

Logical connective: if then

Consider these two simple statements

You’re in army
You’ve to wear uniform.

These are two simple statements. Now I’ll combine these two simple statements (#1 and #2) to form a complex statement.

If you’re in army(#1), you have to wear uniform.(#2)

What about its reverse?

You’ve wearing uniform (#2)—> that means you’re in the army.(#1)
But there is possibility, you’re in navy—-> you’ll still have to wear a uniform. It means,
if 1=>2, then 2=>1 is not always a valid inference.
Let’s list all such scenarios in a table.

Given statement:If you’re in army(#1), you have to wear uniform.(#2)
Inference?		Valid / invalid?
If #2, then #1	If you’ve to wear uniform, you’re in army.	you’ve to wear uniform in navy, air force, BSF etc. so this inference is not always valid.
If not #1, then not #2	if you’re not in army, you don’t have to wear uniform.	you’ve to wear uniform in navy, air force, BSF etc. so this inference is not always valid.
if not #2, then not #1	If you don’t have to wear uniform, you’re not in army.	Always valid.

In the exam, you don’t have to think ^that much. Just mugup the following rule:
Given statement =“If #1 then #2”, in such situation the only valid inference is “if Not #2, then not #1”.
In other words, “if 1^st happens then 2^nd happens”, in such situation, the only valid inference is “if 2^nd did not happen then 1^st did not happen”.
Now I want to construct a short and sweet reference table for the logical connective problems. So I’ll use the symbol ~= negative.

~1=meaning NOT 1 ( or in other words, negative of #1)

Given	Valid inference
If 1, then 2	If not 2, then not 1
If 1=>2	~2=>~1

In some books, material, sites, you’ll find these rules explained as using “P” and “Q” instead of 1 and 2.
But in our method, you first make sure the given (complex) statement starts with a logical connective (or you exchange position as explained earlier)
We denote the first simple sentence as #1 and second simple sentence as #2.
The reason for using 1 and 2= makes things less complicated and easier to mugup.

Logical connective: Only IF

In such scenario, you’ve to rephrase given statement into “if then” and then apply the logical connective rule for “if then”.
For example: given statement: he scores a century, only if the match is fixed.
The “standard format”= only if the match is fixed(1), he scores a century(2).
In case of “only if”, we further convert it into an “if” statement, by exchanging positions. That is
if he scores a century(#2), the match is fixed(#1).
Then apply the formula for “if then” and get valid inference.
Here we’ve “if 2=>1” as per our formula for “if then”, the valid inference will be ~1=>~2. Don’t confuse between 1 and 2. Because essentially the valid inference is “negative of end part => negative of starting part”.
Therefore “if 2=>1 then ~1=~2”
similarly “if 98=>97, then valid inference will be ~97=>~98”
Similarly “if p=>q, then valid inference will be ~q=>~p”,
similarly “if b=>a, then valid inference will be ~a=~b”) .
Update our table

Logical connective	Given statement	Valid inference using symbol	Valid inf. In words
If	If 1=>2	~2=>~1	Negative of end part=> negative of start part
Only if	Only if 1=>2	~1=>~2	Negative of start part=>negative of end part.

Logical Connective: UNLESS

Given statement: Unless you bribe the minister(#1), you will not get the 2G license.(#2)
Unless = if…..not.
So, I can re-write the given statement as
(new) Given statement: If you don’t bribe the minister(#1), you’ll not get the 2G license.(#2)

How to come up with a valid inference here?

#1	You don’t bribe the minister
#2	You’ll not get the 2G license.

For “if..then”, We’ve mugged up the rule: 1=>2 then only valid inference is ~2=>~1. (in other words, negative of end part => negative of starting part).
let’s construct the valid inference for this 2G minister.
we want ~2 => ~1
Negative of (2) => negative of (1)
Negative of (you’ll not get the 2G license)=>negative of (you don’t bribe the minister)
You’ll get the 2G license => you bribe the minister.
In other words, If I see a 2G license in your hand, then I can infer that you had definitely bribed the minister.
This is one way of doing “unless” questions = via converting it into “if…not” type of statement.
The short cut is to mugup another formula: unless1=>2 then ~2=>1.
How did we come up with above formula?

Deriving the formula for unless

Unless 1=>2 (given statement)
if not 1=>2 (because unless=if not)
if ~1=>2 (I’m using symbol ~ instead of “not”)
~2=> ~(~1) (because we already mugged up the rule “if 1=>2, then valid inference is ~2=>~1)
~2=>1 (because ~(~1) means double negative and double negative is positive hence ~(~1)=1)

This is our second rule: Unless1=>2 then ~2=>1

Table

Logical connective	Given statement	Valid inference using symbol	Valid inf. In words
If	If 1=>2	~2=>~1	Negative of end part=> negative of start part
Only if	Only if 1=>2	~1=>~2	Negative of start part=>negative of end part.
Unless	Unless 1=>2	~2=>1	Negative of end part=>start part unchanged.

Logical connective: otherwise

Suppose given statement is: 1, otherwise 2.
you can write it as unless 1 then 2. (unless1=>2)
Then use the formula for “unless.”

Logical connective: When, Whenever, every time

Given statement: he scores century, when match is fixed.
This is not in standard format of “**logical connective word**, simple statement #1, simple statement #2.”
So first I need to exchange the positions: “when match is fixed (#1), he scores century (#2)”.
In case of when and whenever, the valid inference is= same like “If, then”. That means negative of end part=>negative of starting part.
Same formula works for “whenever” and “Everytime”.
Update the table

Logical connective	Given statement	Valid inference using symbol	Valid inf. In words
If	If 1=>2	~2=~1	Negative of end part=> negative of starting part
When	When 1=>2
Whenever	Whenever 1=>2
Everytime	Everytime 1=>2
Only if	Only if 1=>2	~1=>~2	Negative of start part=>negative of end part.
Unless	Unless 1=>2	~2=>1	Negative of end part=>starting part unchanged.

Logical Connective: Either OR

Given statement: Either he is drunk(1) or he is ill(2).

In such cases, if not 1 then 2. And if not 2 then 1.

Meaning,

if he is not drunk then he is definitely ill
if he is not ill, then he is definitely drunk

both are valid. Update the table

Logical connective	Given statement	Valid inference using symbol	Valid inf. In words
If	If 1=>2	~2=~1	Negative of end part=> negative of starting part
When	When 1=>2
Whenever	Whenever 1=>2
Everytime	Everytime 1=>2
Only if	Only if 1=>2	~1=>~2	Negative of start part=>negative of end part.
Unless	Unless 1=>2	~2=>1	Negative of end part=>starting part unchanged.
Otherwise	1 otherwise 2=> rewrite as Unless1=>2.	~2=>1	Negative of end part=>starting part unchanged.
Either or	Either 1 or 2	~2=>1 ~1=>2	Negative of any one part=> remaining part remains unchanged.

Now let’s solve some questions from old CSAT and CAT papers
Please note: in the exam, actual wording / meaning of the simple statement doesn’t matter. Just apply the formulas as given in above table.

For example, “if you’re in army, you have to wear uniform.” Then valid inference is ~2=>~1 (you don’t have to wear uniform, then you’re not in army).
Now ofcourse there would be exceptional situation when army officer/jawan doesn’t need to wear uniform, for example during espionage mission behind the enemy lines. In that case you don’t have to wear uniform, but you’re still in the army.
But keep in mind, while solving logical connective question under the “aptitude/reasoning” portion you don’t have to surgically dissect or nitpick the meaning every statement. Just “if 1=>2” then “~2=>~1”.

Demo Q: Only if: bored TV brother (CSAT 2012)

Examine the following statements:

I watch TV only if I am bored
I am never bored when I have my brother’s company.
Whenever I go to the theatre I take my brother along.

Which one of the following conclusions is valid in the context of the above statements?

If I am bored I watch TV
If I am bored, I seek my brother’s company.
If I am not with my brother, then I’ll watch TV.
If I am not bored I do not watch TV.

Approach

First we’ll construct valid inferences from the question statements

Given Question Statement #1:

Given =I watch TV only if I am bored
This is not in standard format. So first exchange position
Only if I’m bored (1), I watch TV(2)
What is the valid inference? Just look at the formula table
Only if 1=>2 then ~1=~2
Valid inference= if I’m not bored, I do not watch TV.
Look at the statements given in the answer choices, (D) matches. Therefore, final answer is (D).

Demo Q (If, then) Professor Headaches (CAT’98)

You’re given a statement, followed by four statements labeled A to D. Choose the ordered pair of statements where the first statement implies the second and two statements are logically consistent with the main statement.

Given statement: If I talk to my professors(1), then I didn’t need to take a pill for headache.(2)

Four Statements

I talked to my professors
I did not need to take a pill for headache
I needed to take a pill for headache
I did not talk to my professor.

Answer choices

AB
DC
CD
AB and CD

Approach

Given statement is in standard format already

#1	I talk to my professors
#2	I didn’t need to take a pill for headache.

Let’s classify the four statements

Classification	Four statements
1	I talked to my professors
2	I did not need to take a pill for headache
~2	I needed to take a pill for headache
~1	I did not talk to my professor.

Answer choice (i) AB

If you observe the answer choice (I): AB= I talked to my professors, I did not need to take a pill for headache. This is valid because if 1=>2 is already given in the question statement itself.

Answer choice (ii) DC

I did not talk to my professor (~1), I needed to take a pill for headache (~2). Meaning ~1=>~2.
This is invalid because as per our table, if 1=>2, then valid inference is ~2=>~1.

Answer choice (iii) CD

I needed to take pill for headache (~2), I did not talk to my professor (~1). Meaning ~2=>~1. This is valid as per our table. Therefore final answer is (IV) AB and CD

Demo Q: Either or: derailed/late train (CAT’97)

Given statement: either the train is late (1) or it has derailed (2)

Four statements

Train is late = 1
Train is not late = ~1
Train is derailed =2
Train is not derailed =~2

(^note: I’ve classified the statements in advance)

Answer choice

Approach

As per our table, the valid inferences for either or are

~2=>1	If the train is not derailed, it is late.	DA
~1=>2	If the train is not late, it is derailed	BC

Correct answer is (III): BC
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