Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

LSAT - Logical Reasoning Fundamentals > Conditional Logic - Basics > Flashcards

Conditional Logic - Basics Flashcards

Develop deep understanding of the fundamental of conditional logic and diagramming.

1
Q

DEFINE:

Conditional relationship

A

A relationship between two conditions where the truth of one is SUFFICIENT for the truth of the other

OR

A relationship between two conditions where the truth of one is NECESSARY for the truth of the other

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

DEFINE:

Sufficient condition

A

The condition that, if true, GUARANTEES the truth of the other condition

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

DEFINE:

Necessary condition

A

The condition that is REQUIRED in order for the other condition to be true

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

FILL IN THE BLANKS:

___________ condition —> ____________ condition

A

SUFFICIENT condition —> NECESSARY condition

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

VISUALIZE:

Venn diagram form of “If A, then B”

A

A is the SUFFICIENT condition and B is the NECESSARY condition.

The SUFFICIENT condition circle falls entirely within the NECESSARY condition circle.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

DEFINE:

Valid inference / Valid conclusion

A

Something that MUST BE TRUE based on the truth of another statement.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

DEFINE:

Invalid inference / Invalid conclusion

A

Something that does NOT have to be true based on the truth of another statement.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

DEFINE:

Contrapositive

A

Valid inference that because the necessary condition is false, the sufficient condition must be false.

Examples:

Premise: A -> B

Conclusion: /B -> /A

Premise: A -> B

Premise: /B

Conclusion: /A

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

How do we take the CONTRAPOSITIVE of a conditional statement?

A

SWITCH both sides of the arrow AND NEGATE each term

A -> B

Contrapositive: /B -> /A

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

DEFINE:

Converse fallacy

A

Invalidly inferring that because the necessary condition is true, the sufficient condition must be true.

Examples:

Premise: A -> B

Conclusion: B -> A

Premise: A -> B

Premise: B is true.

Conclusion: A is true.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

DEFINE:

Inverse fallacy

A

Invalidly inferring that because the sufficient condition is false, the necessary condition must be false.

Examples:

Premise: A -> B

Conclusion: /A -> /B

Premise: A -> B

Premise: /A

Conclusion: /B

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

TRUE OR FALSE:

We should diagram most conditional statements that we read in the Logical Reasoning section.

A

FALSE.

Diagramming conditionals is the most useful when the problem is about CONNECTING multiple conditional statements OR when there is difficult conditional language/structure.

Read more of the stimulus first before deciding whether and what to diagram.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

DIAGRAM:

X if Y.

A

Y -> X

/X -> /Y

IF introduces a SUFFICIENT condition.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

DIAGRAM:

L only if S.

A

L -> S

/S -> /L

ONLY IF introduces a NECESSARY condition.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

DIAGRAM:

Only J are F.

A

F -> J

/J -> /F

ONLY introduces a NECESSARY condition.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

DIAGRAM:

The only A are Q.

A

A -> Q

/Q -> /A

THE ONLY introduces a SUFFICIENT condition.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

DIAGRAM:

R if and only if L.

A

R L

/L /R

IF AND ONLY IF creates a BICONDITIONAL relationship. Which side the concepts are on doesn’t matter, since BOTH concepts are SUFFICIENT and NECESSARY.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

DIAGRAM:

Y unless R.

A

/R -> Y

/Y -> R

UNLESS introduces SUFFICIENT, BUT NEGATED.

Alternate method: UNLESS introduces NECESSARY, but the NEGATE OTHER CONCEPT.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

DIAGRAM:

No Z is W.

A

Z -> /W

W -> /Z

NO introduces SUFFICIENT, but NEGATE OTHER CONCEPT.

Alternate method: NO introduces NECESSARY, BUT NEGATED.

20
Q

DIAGRAM:

T is essential for F.

A

F -> T

/T -> /F

Something that is ESSENTIAL is NECESSARY.

21
Q

DIAGRAM:

Y is a precondition for E.

A

E -> Y

/Y -> /E

A PRECONDITION is NECESSARY.

22
Q

DIAGRAM:

Every K is an N.

A

K -> N

/N -> /K

EVERY introduces a SUFFICIENT condition.

23
Q

DIAGRAM:

Only if there is W can there be Q.

A

Q —> W

/W —> /Q

ONLY IF introduces the NECESSARY condition.

24
Q

DIAGRAM:

V is the only way to A.

A

A -> V

/V -> /A

THE ONLY introduces the SUFFICIENT condition.

25
Q

DIAGRAM:

T cannot occur without J.

A

/J —> /T

T —> J

WITHOUT is like UNLESS: introduces SUFFICIENT, BUT NEGATED.

Alternate method: WITHOUT introduces NECESSARY, but NEGATE THE OTHER CONCEPT.

26
Q

TRUE OR FALSE:

A conditional statement and its contrapositive are logically equivalent to each other.

A

TRUE.

The contrapositive is just another way to express the exact same conditional relationship.

(So if you diagram the contrapositive of a statement when an explanation diagrammed it the other way, that’s OK.)

27
Q

DIAGRAM:

G requires H.

A

G —> H

/H —> /G

REQUIRES introduces a NECESSARY condition.

(Note that “G requires H” = “H is required for G.”)

28
Q

DIAGRAM:

T, except L.

A

/L —> T

/T —> L

EXCEPT is like UNLESS. It introduces SUFFICIENT, BUT NEGATED.

Alternate method: EXCEPT introduces NECESSARY, but NEGATE THE OTHER CONCEPT.

29
Q

DIAGRAM:

Y are B.

A

Y —> B

/B —> /Y

A basic sentence can be diagrammed with the SUBJECT as the SUFFICIENT condition and the PREDICATE as the NECESSARY condition.

30
Q

DEFINE:

Subject (grammar)

A

The noun or noun phrase that is doing the action expressed by the verb.

Example:

“The [hard-working students who mastered these flashcards] felt their brains get bigger.”

The bracketed phrase is the SUBJECT which is doing the action: feeling their brains get bigger.

31
Q

DEFINE:

Predicate (grammar)

A

The part of a statement that describes a property of the subject or that characterizes the subject.

Example:

“The hard-working students who mastered these flashcards [felt their brains get bigger.]”

The bracketed phrase is the PREDICATE: it is something that is true about the subject of the sentence.

32
Q

DIAGRAM:

Tim will go to the party only if his friends go.

A

Tim goes to party —> His friends go to the party

His friends do NOT go to the party —> Tim does NOT go to the party.

ONLY IF introduces the NECESSARY condition (the “then” part of an If-then statement.)

33
Q

DIAGRAM:

The plague will not stop unless we sacrifice 20 goats.

A

If we do NOT sacrifice 20 goats —> Plague will NOT stop

Plagus stops —> Sacrifice 20 goats

UNLESS introduces the SUFFICIENT, BUT NEGATED.

Alternate method: UNLESS introduces NECESSARY but NEGATE THE OTHER CONCEPT.

34
Q

DIAGRAM:

Colonizing the moon is the only way to prevent overpopulation of the Earth.

A

Prevent overpopulation of Earth —> Colonize the moon

If we do NOT colonize the moon —> CANNOT prevent overpopulation of Earth

THE ONLY introduces the SUFFICIENT condition.

35
Q

DIAGRAM:

None of the students who were late were qualified for extra credit.

A

If student was late —> NOT qualified for extra credit

If qualified for extra credit —> NOT student who was late

NONE is like NO. It intoduces the SUFFICIENT, BUT NEGATE OTHER CONCEPT.

Alternate method: NONE introduces NECESSARY BUT NEGATED.

36
Q

DIAGRAM:

Only candidates who pander to the masses can win an election.

A

If win election —> Pander to masses

If NOT pander to masses —> NOT win election

ONLY introduces a NECESSARY condition.

37
Q

DIAGRAM:

Technological development will not slow until society changes its values.

A

If society does NOT change its value —> Tech. dev. will NOT slow down

If tech dev. slows down —> Society changed its values

UNTIL is like UNLESS. It introduces SUFFICIENT, BUT NEGATED.

Alternate method: Until introduces NECESSARY, but NEGATE THE OTHER CONCEPT.

38
Q

DIAGRAM:

I cry when I am sad.

A

Sad —> Cry

NOT Cry —> NOT Sad

WHEN introduces the SUFFICIENT condition.

39
Q

DIAGRAM:

Mastering conditional logic is essential to a high LSAT score.

A

If High LSAT score —> Master conditional logic

If NOT master conditional logic —> NOT high LSAT score

Something that is ESSENTIAL is the NECESSARY condition.

40
Q

DIAGRAM:

Without adding more butter, the cake will not have a strong taste.

A

If NOT add more butter —> Cake will NOT have strong taste

If cake has strong taste —> Added more butter

WITHOUT is like UNLESS. It introduces the SUFFICIENT, BUT NEGATED.

Alternate method: WITHOUT introduces NECESSARY, but NEGATE THE OTHER CONCEPT.

41
Q

DIAGRAM:

The illness affected all those who ate the raw fish.

A

If ate raw fish —> Illness

If NOT illness —> did NOT eat raw fish

ALL introduces a SUFFICIENT CONDITION.

42
Q

DIAGRAM:

To develop a healthy immune system, one must take vitamin supplements daily.

A

If healthy immune system —> Takes vitamin supplements daily

If NOT takes vitamin supplements daily —> NOT healthy immune system

MUST introduces a NECESSARY condition.

43
Q

DIAGRAM:

Ellen will appear in the show if and only if the lead actress gets injured.

A

Ellen appears in show <—> Lead actress injured

Ellen is NOT in show <—> Lead actress NOT injured

IF AND ONLY IF introduces a BICONDITIONAL. BOTH concepts are SUFFICIENT and NECESSARY.

(You don’t have to switch the sides for the contrapositive of a biconditional, because the arrow goes both ways.)

44
Q

VALID ARGUMENT?

Our profits will increase only if we raise our prices. We just raised our prices. Therefore, our profits will increase.

A

INVALID

ONLY IF introduces the NECESSARY condition.

Raising prices is NECESSARY in order to increase profits. But it doesn’t guarantee that profits will increase. Maybe we raise our prices but there’s a global pandemic that strikes and hurts our business.

45
Q

VALID ARGUMENT?

Raymond will not enjoy steak unless it is well-done. This steak is well-done. So, Raymond will enjoy it.

A

INVALID

The first premise: If steak is NOT well-done —> Raymond will NOT enjoy it.

But even if it IS well-done, he still might not enjoy it. (What if it’s too small or spoiled or has bad seasoning, etc.?)

(You can also view the first premise like this: If Raymond enjoys steak —> it IS well-done.)

46
Q

VALID ARGUMENT?

Daily flossing is required in order to maintain healthy gums. The feral child raised by wolves does not floss daily. Hence, he must not have healthy gums.

A

VALID — CONTRAPOSITIVE

The first premise tells us that daily flossing is NECESSARY in order to maintain healthy gums. So if the child doesn’t do what’s necessary, he won’t have healthy gums.

LSAT - Logical Reasoning Fundamentals (10 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions