👁️🗨️Formal Logic I Unit 12 – Natural Deduction in Predicate Logic
Natural deduction is a formal logic system used to prove argument validity through inference rules. It mimics natural reasoning, starting with premises and deriving conclusions step-by-step, using logical connectives and quantifiers in both propositional and predicate logic.
This method provides a foundation for formal proofs in math, philosophy, and computer science. Key concepts include premises, conclusions, inference rules, logical connectives, quantifiers, and the distinction between free and bound variables within a specified domain.
System of formal logic used to prove the validity of arguments based on a set of inference rules
Starts with premises and uses rules of inference to derive conclusions step-by-step
Differs from other proof systems like truth tables or resolution by mimicking the natural reasoning process
Proofs are constructed as a sequence of lines, each line being derived from previous lines using inference rules
Allows for the introduction and elimination of logical connectives (and, or, if-then) and quantifiers (for all, there exists)
Provides a foundation for understanding formal proofs in mathematics, philosophy, and computer science
Can be applied to both propositional logic and predicate logic, with additional rules for handling quantifiers in predicate logic
Key Concepts and Terminology
Premise: a statement that is assumed to be true for the purpose of the argument
Conclusion: the statement that is derived from the premises using rules of inference
Inference rule: a logical rule that allows new lines to be derived from previous lines in a proof
Logical connectives: symbols used to combine propositions, such as ∧ (and), ∨ (or), → (if-then), and ¬ (not)
Quantifiers: symbols used to express the quantity of elements in a domain that satisfy a given predicate, such as ∀ (for all) and ∃ (there exists)
Scope: the portion of a formula that a quantifier applies to, usually denoted by parentheses or brackets
Free variable: a variable that is not bound by a quantifier and can take on any value from the domain
Bound variable: a variable that is bound by a quantifier and ranges over the specified domain
Domain: the set of elements that variables can take values from in a given context
Rules of Inference
Modus Ponens (MP): if P and P→Q are true, then Q is true
Modus Tollens (MT): if P→Q and ¬Q are true, then ¬P is true
Hypothetical Syllogism (HS): if P→Q and Q→R are true, then P→R is true
Disjunctive Syllogism (DS): if P∨Q and ¬P are true, then Q is true
Constructive Dilemma (CD): if (P→Q)∧(R→S) and P∨R are true, then Q∨S is true
Conjunction Introduction (Conj): if P and Q are true, then P∧Q is true
Conjunction Elimination (Simp): if P∧Q is true, then P is true and Q is true
Disjunction Introduction (Add): if P is true, then P∨Q is true for any Q
Biconditional Introduction (Bicond): if P→Q and Q→P are true, then P↔Q is true
Quantifiers and Their Role
Universal quantifier (∀): expresses that a predicate holds for all elements in a domain
Example: ∀x(P(x)→Q(x)) means for all x, if P(x) is true, then Q(x) is true
Existential quantifier (∃): expresses that a predicate holds for at least one element in a domain
Example: ∃xP(x) means there exists at least one x such that P(x) is true
Quantifiers allow for reasoning about properties of entire sets or domains rather than just individual elements
The order of quantifiers matters, as it determines the dependencies between variables
Example: ∀x∃yP(x,y) is different from ∃y∀xP(x,y)
Quantifier negation rules:
¬(∀xP(x))≡∃x¬P(x)
¬(∃xP(x))≡∀x¬P(x)
Quantifier instantiation: replacing a universally quantified variable with a specific term or constant
Existential generalization: introducing an existential quantifier when a statement holds for a specific element
Constructing Proofs
Begin by clearly stating the premises and the conclusion to be proved
Work backwards from the conclusion, looking for inference rules that can be applied to derive it
Introduce assumptions as needed, using the premise lines or derived lines
Apply inference rules to derive new lines from the premises and previously derived lines
Use quantifier rules to instantiate variables or introduce new variables as needed
Ensure that each line is properly justified by citing the inference rule and the line numbers it depends on
Continue the process until the conclusion is derived, at which point the proof is complete
Review the proof for clarity, correctness, and conciseness, making any necessary revisions
Label the proof with a title or description for easy reference
Common Proof Strategies
Direct proof: assume the premises and use inference rules to derive the conclusion directly
Proof by contradiction: assume the negation of the conclusion and show that it leads to a contradiction with the premises or known facts
Also known as reductio ad absurdum
Proof by cases: break the problem into distinct cases and prove the conclusion for each case separately
Useful when the conclusion depends on different conditions or scenarios
Existential instantiation: when proving a statement with an existential quantifier, introduce a new constant to represent the element that satisfies the predicate
Universal generalization: when a statement has been proved for an arbitrary element, conclude that it holds for all elements in the domain
Hypothetical proof: assume an additional premise temporarily and use it to derive the conclusion, then discharge the assumption using the Deduction Theorem
Proof by induction: prove a statement for a base case and then show that if it holds for one case, it must hold for the next case, concluding that it holds for all cases
Pitfalls and How to Avoid Them
Forgetting to consider all possible cases or scenarios when using proof by cases
Ensure that the cases are exhaustive and mutually exclusive
Misusing quantifiers or confusing their order, leading to incorrect statements
Pay close attention to the scope and order of quantifiers, using parentheses to clarify when needed
Introducing unnecessary assumptions or variables that complicate the proof
Aim for a concise and straightforward proof, only introducing elements that are essential to the argument
Failing to properly justify lines or cite the inference rules used
Each line should be clearly justified by citing the inference rule and the line numbers it depends on
Confusing implication (→) with biconditional (↔), or necessary conditions with sufficient conditions
Remember that implication is a one-way relationship, while biconditional is a two-way relationship
Skipping steps or making leaps in reasoning that are difficult for others to follow
Break down the proof into smaller, more manageable steps to ensure clarity and understanding
Misinterpreting the meaning of logical connectives or quantifiers in the context of the problem
Carefully consider the intended meaning of each symbol and how it relates to the problem at hand
Practical Applications
Analyzing arguments in philosophy, law, and everyday reasoning to determine their validity and soundness
Verifying the correctness of mathematical proofs and theorems
Designing and validating algorithms in computer science and artificial intelligence
Modeling and reasoning about complex systems in fields such as economics, biology, and social sciences
Developing formal specifications and verifying the properties of software and hardware systems
Creating and evaluating logical frameworks for decision-making and problem-solving in various domains
Teaching critical thinking and logical reasoning skills to students in mathematics, philosophy, and other disciplines
Contributing to the development of automated theorem provers and proof assistants for use in research and industry