In our previous post we discussed about
the concept of proposition and logical deduction techniques. Here we discuss
about the important rules for deriving conclusion from two given premises.
RULES FOR DERIVING
CONCLUSION FROM TWO GIVEN PREMISES:
1. The conclusion does
not contain the middle term.
Example
Statements:
1.
All men are girls.
2.
Some girls are students.
Conclusions:
1.
All girls are men.
2.
Some girls are not students.
Since
both the conclusions 1 and 2 contain the middle term 'girls', so neither of
them can follow
2. No term can be distributed in the conclusion unless
it is distributed in the premises.
Example
Statements:
1.
Some dogs are goats.
2. All goats are cows.
Conclusions:
1. All cows are goats.
2. Some dogs are cows.
Statement 1 is an I-type proposition
which distributes neither the subject nor the predicate.
Statement 2 is an A type proposition
which distributes the subject i.e. 'goats' only.
Conclusion 1 is an A-type proposition
which distributes the subject 'cow' only since the term 'cows' is distributed
in conclusion 1 without being distributed in the premises, so conclusion 1
cannot follow.
3. The middle term (M) should he distribute at least once in the
premises. Otherwise, the conclusion cannot follow.
For
the middle term to be distributed in a premise
(i) M must be the
subject if premise is an A proposition.
(ii)
M must be subject or predicate if premise is an E proposition.
(iii)
M must be predicate if premise is an O proposition.
Note that in an
I
proposition, which distributes neither the subject nor the predicate, the
middle term cannot be distributed.
Example
Statements:
1.
All fans are watches.
2.
Some watches are black.
Conclusions:
1.
All watches are fans.
2.
Some fans are black.
In the premises, the middle term is
'watches'. Clearly, it is not distributed in the first premise which is an A
proposition as it does not form its subject. Also, it is not distributed in the
second premise which is an I proposition. Since the middle term is
not distributed even once in the premises, so no conclusion follows.
4. No conclusion follows
(a) if both the
premises are particular
Example
Statements:
1.
Some books are pens.
2.
Some pens are erasers.
Conclusions:
1.
All books are erasers.
2.
Some erasers are books.
Since
both the premises are particular, so no definite conclusion follows.
(b)
If both the premises are negative.
Example
Statements:
1.
No flower is mango.
2.
No mango is cherry.
Conclusions:
1.
No flower is cherry.
2.
Some cherries are mangoes. Since both the premises are negative, neither
conclusion follows.
(c)
If the major premise is particular and the minor premise is negative.
Example
Statements:
1.
Some dogs are bulls.
2.
No tigers are dogs.
Conclusions:
1.
No dogs are tigers.
2.
Some bulls are tigers.
Here, the first premise containing the
middle term 'dogs' as the subject is the major premise and the second premise containing
the middle term 'dogs' as the predicate is the minor premise. Since the major
premise is particular and the minor premise is negative, so no conclusion
follows.
5. If the middle term is
distributed twice, the conclusion cannot be universal.
Example
Statements:
1.
All fans are chairs.
2.
No tables are fans.
Conclusions:
1.
No tables are chairs.
2.
Some tables are chairs.
Here, the first premise is an A
proposition and so, the middle term 'fans' forming the subject is distributed.
The second premise is an E proposition and so, the middle term 'fans' forming
the predicate is distributed. Since the middle term is distributed twice, so
the conclusion cannot be universal.
6. If one premise is negative, the conclusion must be negative.
Example
Statements:
1.
All grasses are trees.
2.
No tree is shrub.
Conclusions:
1. No grasses are shrubs.
2. Some shrubs are grasses.
Since one premise is negative, the
conclusion must be negative. So, conclusion 2 cannot follow.
7. If one premise is particular, the conclusion must be
particular.
Example
Statements:
1.
Some boys are thieves.
2.
All thieves are dacoits.
Conclusions:
1.
Some boys are dacoits.
2.
All dacoits are boys.
Since
one premise is particular, the conclusion must be particular. So, conclusion 2
cannot follow.
8. If both the premises are affirmative, the conclusion must be
affirmative.
Example
Statements:
1.
All women are mothers.
2.
All mothers are sisters.
Conclusions:
1.
All women are sisters.
2.
Some women are not sisters.
Since both the premises are
affirmative, the conclusion must be affirmative. So, conclusion 2 cannot
follow.
9. If both the premises
are universal, the conclusion must be universal.
Complementary pair:
A pair of contradictory statements i.e.
a pair of statements such that if one is true, the other is false and when no
definite conclusion can be drawn, either of them is bound to follow, is called
a complementary pair. E and I-type propositions together form a complementary
pair and usually either of them follows, in a case where we cannot arrive at a
definite conclusion, using the rules of syllogism.
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