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13.5. Question 4
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Today, we're going to delve into implications with quantified statements. Let's review what it means for property P to be true for some element and property Q to be true for a potentially different element. Can anyone tell me what that suggests about their relationship?
It means they could be true for different elements in the domain, right?
Exactly! So we cannot assume a single element satisfies both. This leads us to our first key point: just because both properties are true for some element doesn't mean they are true for the same element.
Could you give an example to illustrate that?
Certainly! If we say P(x) 'is even' is true for x = 2 and Q(y) 'is prime' is true for y = 3, it doesn't mean there's an element that is both even and prime!
So we can't just jump to conclusions about shared elements.
Exactly! This is a crucial aspect of understanding logical statements.
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Let’s explore an actual counterexample now. Imagine we have two predicates, P and Q. How can we construct an argument that shows the failure of our initial implication?
Maybe we choose a domain with two elements? One where P holds, and another where Q holds?
That’s a great approach! If we let P be true for x1 but false for x2, and Q true for x2 but false for x1, we see no single x satisfies both. Would anyone like to simplify this conclusion?
So, it illustrates the limitation of our initial assumption regarding shared elements?
Correct! This illustrates that just because both exist, doesn't mean they common. Let's remember that!
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Now let's shift to the other direction of our discussion. If I say there exists an x for which both P(x) and Q(x) are true, what can we conclude from this?
We can say that both properties P and Q hold independently at least for that x?
Exactly! This is known as existential instantiation, where we take our common x and assert both properties hold independently. How can we prove that?
We show that instance c works for both P and Q!
Spot on! That's how we confirm our implication is valid in this case.
Overview
Short Summary
In this section, we explore implications involving quantified expressions, specifically their validity and how to prove or disprove them.
Medium Summary
This section focuses on the analysis of implications in logic involving existential and universal quantifiers. We demonstrate how to construct valid arguments using examples. A counterexample illustrates the invalidity of an argument, while the discussion shows the relationship between different quantifiers. Furthermore, we differentiate between the two forms of implication regarding existential statements.
Detailed Summary
Detailed Summary
In this section, the focus is on understanding implications involving quantified statements in logic. The section begins by exploring an argument that states if a property P is true for some element of a domain and another property Q is true for some element of the domain, then can we conclude that there is a common element for which both properties hold. This is demonstrated through a counterexample, proving the fallacy of this implication.
The counterexample involves defining two predicates P and Q for different elements in a domain where P is true for one and Q is true for another, yet no single element satisfies both predicates. The discussion elaborates on using existential quantifiers and how their independence means that the implication does not hold universally. Subsequently, the focus shifts to a different implication, where if both properties are true for a single x in the domain, it can be concluded that both properties are true independently, confirmed through proof strategies, including existential instantiation.
This section illustrates the importance of understanding logical implications involving quantifiers and reinforces how subtle distinctions can determine the validity of logical statements.
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Audio Book
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Create a free accountHere I have to show or I have to either prove or disprove that the left hand side expression implies the right hand side expression. So you see the left hand side expression, I have explicitly added the parenthesis here, so the x within the P and x within the Q are different here whereas in the right hand side the x both within P and Q are the same. Because both of them are covered by the same ‘there exist’.
Detailed Explanation
In this chunk, we are discussing an implication between two expressions involving quantifiers. We have two statements: the left-hand side (LHS) involves two different instances of 'x' for predicates P and Q, while the right-hand side (RHS) considers the same 'x'. This difference is crucial because implications can vary significantly based on whether they deal with the same or different instances of a variable. The LHS states that for some x, P is true, and for some potentially different x, Q is true. The RHS, however, asserts that there exists an x for which both P and Q are true simultaneously.
Examples & Analogies
Imagine a scenario in a class of students: The LHS suggests 'some student is good at math' while 'some student is good at art,' but they could be different students. The RHS asserts 'there is a student who is good at both math and art.' It’s possible that the students who excel at each subject are different, leading to the conclusion that a student exists who excels in both may not be valid.
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Create a free accountWhereas, in the left hand side, the first x is covered by the first ‘there exist’ (ꓯ ) and the second x is covered by the second ‘there exist’ (ꓯ ). The informal way to interpret the statement is if you are given that property P is true for some element in the domain and if you are given that property Q is true for some element of the domain, then can you conclude that both P and Q property are true for some element of the domain.
Detailed Explanation
In this section, we are analyzing why the implication from the LHS to the RHS does not always hold true. By using counterexamples, we can illustrate that even if there are students who are good at math (P) and good at art (Q), they need not be the same individual. Thus, the conclusion that there exists a student proficient in both subjects cannot always be drawn.
Examples & Analogies
Imagine a basketball team where we know that 'some player can shoot three-pointers' and 'some player can dunk' — these can be two different players. Just because one excels at shooting doesn’t guarantee that the same player also excels in dunking, which creates a situation where the conclusion doesn't follow from the premises.
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Create a free accountWhat about the part b is the implication in the reverse direction. It says that if you are given that ꓯ some x value in the domain for which both property P and property Q are true. Then you can conclude that individually the property P and Q are true for some value in the domain.
Detailed Explanation
Here we explore the converse statement: if we assume that there exists an x in the domain for which both P and Q are true, we can conclude that there must be at least one instance where P is true and at least one instance where Q is also true. This assertion correctly follows from the definition of existential quantification.
Examples & Analogies
Returning to our basketball team, if we assert 'there is a player who can do both a three-pointer and a dunk', we can reasonably conclude that there exists a player who can shoot three-pointers, as well as a different player who can dunk. This follows logically from our initial premise.
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Create a free accountSo to prove that this implication is true, we have to show that if I assume left hand side is true, then I have to show that the right hand side is also true.
Detailed Explanation
In conclusion, we establish the truth of implications through logical reasoning. By assuming the LHS is true and demonstrating that this leads to the truth of the RHS, we validate the logical structure of the statements. The proof can follow through substitutions and reasoning about their respective elements in the structure of the domain.
Examples & Analogies
Think of this like a road map: if you're already at a town (LHS), following the road signs (the logical implications) guarantees that you will eventually reach your destination (RHS). Therefore, if all conditions are met along the way, one will necessarily lead to the other.
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Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- Logical Implication:
The relationship that indicates one statement can lead to the truth of another.
- Existential Quantification:
Expresses the existence of at least one element in a domain.
- Universal Quantification:
States that a property holds for all elements within a domain.
- Counterexample:
An example demonstrating that a particular statement is not universally true.
- Existential Instantiation:
A logical method to establish the truth of specific instances from broad existential statements.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
When defining predicates for properties like 'being even' or 'being prime', examples illustrate how they can hold true for different elements but not simultaneously for the same one.
Using a simple counterexample: P(x) may hold for x = 2 and Q(y) for y = 3, leading to a clear demonstration of when implications fail.
Memory aids
Imagine a town where every person loves math (universal). Then, one day, a new student joins who loves art (existential). Both are happy together, but not both in the same way!
For quantifiers, just remember: E for Existential means 'Exists,' U for Universal means 'All Us!'
Flash Cards
Glossary
Implication
A logical connection between statements that asserts if one statement is true, then the other must also be true.
Existential Quantifier
A quantifier used to express that there exists at least one element in a domain for which a property holds true.
Universal Quantifier
A quantifier used to express that a property or condition holds for all elements in a domain.
Counterexample
An example that disproves a proposition or theory.
Existential Instantiation
The logical process of deducing specific instances from an existential statement.