 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 HeadachesÂ (CATâ98)
 Demo Q: Either or: derailed/late train (CATâ97)
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 âall, none, someâ etc. Can be classified into UP, UN,PP and PN. Already explained in previous articles.  Contains words like âif, unless, only if, wheneverâ etc. can be classified into 1, ~1, 2, ~2 (weâll see in this article) 
Have to mugup more formulas, takes more time than logical connective questions.  Less formulas and quicker than syllogism. 
Question Statements:
Conclusion choices:

Question statements:
Conclusion choices:

Standard format: logical connectives
 If, unless, only if, whenever, every time etc. are examples of Logical connectives.
 Whenever youâre 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 youâre in the army, youâve to wear uniform 

Youâve to wear uniform, if youâre in the army 

Youâve to salute, whenever Commanding Officer comes in your cabin. 

Now letâs derive valid inferences for various logical connectives.
Logical connective: if then
Consider these two simple statements
 Youâre in army
 Youâve to wear uniform.
These are two simple statements. Now Iâll combine these two simple statements (#1 and #2) to form a complex statement.
 If youâre in army(#1), you have to wear uniform.(#2)
What about its reverse?
 Youâve wearing uniform (#2)—> that means youâre in the army.(#1)
 But there is possibility, youâre in navy—> youâll still have to wear a uniform. It means,
 if 1=>2, then 2=>1 is not always a valid inference.
 Letâs list all such scenarios in a table.
Given statement:If youâre in army(#1), you have to wear uniform.(#2)  
Inference?  Valid / invalid?  

If youâve to wear uniform, youâre in army.  youâve to wear uniform in navy, air force, BSF etc. so this inference is not always valid. 

if youâre not in army, you donât have to wear uniform.  youâve to wear uniform in navy, air force, BSF etc. so this inference is not always valid. 

If you donât have to wear uniform, youâre not in army.  Always valid. 
 In the exam, you donât have to think ^that much. Just mugup the following rule:
 Given statement =âIf #1 then #2â, in such situation the only valid inference is âif Not #2, then not #1â.
 In other words, âif 1^{st} happens then 2^{nd} happensâ, in such situation, the only valid inference is âif 2^{nd} did not happen then 1^{st} did not happenâ.
 Now I want to construct a short and sweet reference table for the logical connective problems. So Iâll 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, youâll find these rules explained as using âPâ and âQâ 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, youâve to rephrase given statement into âif thenâ and then apply the logical connective rule for âif thenâ.
 For example: given statement: he scores a century, only if the match is fixed.
 The âstandard formatâ= only if the match is fixed(1), he scores a century(2).
 In case of âonly ifâ, we further convert it into an âifâ statement, by exchanging positions. That is
 if he scores a century(#2), the match is fixed(#1).
 Then apply the formula for âif thenâ and get valid inference.
 Here weâve âif 2=>1â as per our formula for âif thenâ, the valid inference will be ~1=>~2. Donât confuse between 1 and 2. Because essentially the valid inference is ânegative of end part => negative of starting partâ.
 Therefore âif 2=>1 then ~1=~2â
 similarly âif 98=>97, then valid inference will be ~97=>~98â
 Similarly âif p=>q, then valid inference will be ~q=>~pâ,
 similarly âif b=>a, then valid inference will be ~a=~bâ) .
 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 = ifâŚ..not.
 So, I can rewrite the given statement as
 (new) Given statement: If you donât bribe the minister(#1), youâll not get the 2G license.(#2)
How to come up with a valid inference here?
#1  You donât bribe the minister 
#2  Youâll not get the 2G license. 
 For âif..thenâ, Weâve mugged up the rule:Â 1=>2 then only valid inference is ~2=>~1. (in other words, negative of end part => negative of starting part).
 letâs construct the valid inference for this 2G minister.
 we want ~2 => ~1
 Negative of (2) => negative of (1)
 Negative of (youâll not get the 2G license)=>negative of (you donât bribe the minister)
 Youâll 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 âunlessâ questions = via converting it into âifâŚnotâ 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=>2Â (because unless=if not)
 if ~1=>2 (Iâm using symbol ~ instead of ânotâ)
 ~2=> ~(~1) (because we already mugged up the rule âif 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. Â (unless1=>2)
 Then use the formula for âunless.â
Logical connective: When, Whenever, every time
 Given statement: he scores century, when match is fixed.
 This is not in standard format of â**logical connective word**, simple statement #1, simple statement #2.â
 So first I need to exchange the positions: âwhen match is fixed (#1), he scores century (#2)â.
 In case of when and whenever, the valid inference is= same like âIf, thenâ. That means negative of end part=>negative of starting part.
 Same formula works for âwheneverâ and âEverytimeâ.
 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.  
Either or  Either 1 or 2 

Negative of any one part=> remaining part remains unchanged. 
 Now letâs solve some questions from old CSAT and CAT papers
 Please note: in the exam, actual wording / meaning of the simple statement doesnât matter. Just apply the formulas as given in above table.
 For example, âif youâre in army, you have to wear uniform.â Then valid inference is ~2=>~1 (you donât have to wear uniform, then youâre not in army).
 Now ofcourse there would be exceptional situation when army officer/jawan doesnât need to wear uniform, for example during espionage mission behind the enemy lines. In that case you donât have to wear uniform, but youâre still in the army.
 But keep in mind, while solving logical connective question under the âaptitude/reasoningâ portion you donât have to surgically dissect or nitpick the meaning every statement. Just âif 1=>2â then â~2=>~1â.
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 brotherâs 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 brotherâs company.
 If I am not with my brother, then Iâll watch TV.
 If I am not bored I do not watch TV.
Approach
First weâll 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 Iâm 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 Iâm 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 HeadachesÂ (CATâ98)
Youâre 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 didnât 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 didnât need to take a pill for headache. 
Letâs classify the four statements
Classification  Four statements 
1 

2 

~2 

~1 

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 (CATâ97)
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: Iâve classified the statements in advance)
Answer choice
 AB
 DB
 CA
 BC
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
For more articles on reasoning and aptitude, visit Mrunal.org/aptitude