👁️‍🗨️Formal Logic I Unit 8 – Predicate Logic: Symbols and Language

What's This All About?

• Predicate logic builds upon propositional logic by introducing predicates, quantifiers, and variables
• Allows for more expressive and precise logical statements compared to propositional logic
• Variables represent objects or entities in the domain of discourse
• Predicates describe properties of or relationships between objects
• Quantifiers specify the scope and quantity of variables in a statement
• Enables reasoning about complex statements involving multiple objects and their properties
• Provides a foundation for many areas of mathematics, computer science, and philosophy

Key Symbols and Their Meanings

• Predicates: Capital letters (P, Q, R) represent properties or relations
  • Example: P(x) could mean "x is a prime number"
• Variables: Lowercase letters (x, y, z) represent objects in the domain
• Universal quantifier (∀): "For all" or "For every", denotes that a statement holds for all objects in the domain
  • Example: ∀x P(x) means "For all x, P(x) is true"
• Existential quantifier (∃): "There exists" or "For some", denotes that a statement holds for at least one object in the domain
  • Example: ∃x P(x) means "There exists an x such that P(x) is true"
• Logical connectives: ∧ (and), ∨ (or), ¬ (not), → (implies), ↔ (if and only if)
• Parentheses: Used to group subformulas and establish precedence

Building Blocks: Atomic Sentences

• Atomic sentences are the simplest well-formed formulas in predicate logic
• Consist of a predicate followed by a tuple of terms (variables or constants)
• Examples: P(x), Q(a, b), R(x, y, z)
• The arity of a predicate is the number of arguments it takes
  • P(x) is a unary predicate, Q(a, b) is a binary predicate, etc.
• Atomic sentences are the basic units of meaning in predicate logic
• Can be combined using logical connectives and quantifiers to form more complex statements

Putting It Together: Compound Sentences

• Compound sentences are formed by combining atomic sentences using logical connectives and quantifiers
• Examples: ∀x (P(x) ∧ Q(x)), ∃y (R(y) ∨ S(y))
• The scope of a quantifier is the part of the formula it applies to
  • In ∀x (P(x) ∧ Q(x)), the scope of ∀x is (P(x) ∧ Q(x))
• Bound variables are variables that are quantified, while free variables are not
• The order of quantifiers matters and can change the meaning of a statement
  • ∀x ∃y P(x, y) is different from ∃y ∀x P(x, y)
• Nested quantifiers allow for expressing complex relationships between objects

Truth Tables: The Logic Behind It All

• Truth tables are used to evaluate the truth values of compound sentences
• Each row represents a possible combination of truth values for the atomic sentences
• The number of rows in a truth table is 2^n, where n is the number of atomic sentences
• Logical connectives have their own truth tables that determine the output based on the input values
• Quantifiers are evaluated by considering all possible assignments of objects to variables
  • ∀x P(x) is true if P(x) is true for all possible objects in the domain
  • ∃x P(x) is true if P(x) is true for at least one object in the domain
• Truth tables help determine the validity and equivalence of logical statements

Translating Between English and Symbols

• Translating English sentences into predicate logic involves identifying predicates, variables, and quantifiers
• Key phrases for universal quantifiers: "all", "every", "each", "any"
  • "All dogs are mammals" could be translated as ∀x (Dog(x) → Mammal(x))
• Key phrases for existential quantifiers: "some", "there exists", "at least one"
  • "Some birds can fly" could be translated as ∃x (Bird(x) ∧ CanFly(x))
• Negation can be applied to quantifiers or predicates
  • "Not all students are athletes" could be translated as ¬∀x (Student(x) → Athlete(x))
• Multiple quantifiers and logical connectives may be needed for complex sentences
• Translating helps clarify the logical structure of natural language statements

Common Mistakes and How to Avoid Them

• Mixing up the order of quantifiers
  • "Every student has a favorite book" is ∀x (Student(x) → ∃y (Book(y) ∧ FavoriteOf(y, x)))
  • Not ∃y (Book(y) ∧ ∀x (Student(x) → FavoriteOf(y, x)))
• Confusing the scope of quantifiers and logical connectives
  • Use parentheses to clearly delineate the scope
• Forgetting to quantify variables
  • All variables in a formula must be either bound by a quantifier or free
• Misinterpreting the meaning of logical connectives
  • Implication (→) is often confused with biconditional (↔)
• Double-check translations between English and predicate logic
• Break down complex sentences into smaller parts and translate each part separately

Real-World Applications

• Database queries and information retrieval
  • Predicate logic can express complex queries involving multiple entities and relationships
• Artificial intelligence and knowledge representation
  • Predicate logic is used to represent and reason about knowledge in AI systems
• Software specification and verification
  • Predicate logic helps specify the desired behavior of software systems and verify their correctness
• Natural language processing and understanding
  • Predicate logic can capture the underlying logical structure of natural language statements
• Automated theorem proving and reasoning
  • Predicate logic is the basis for many automated theorem provers and reasoning systems
• Mathematical foundations and metamathematics
  • Predicate logic is a fundamental tool for studying the foundations of mathematics and logic itself