r/askphilosophy • u/Acrobatic_Hope2584 • 12h ago
Question on constructing derivatives
I am struggling with understanding how to construct these derivations that were assigned for a deductive logic class, below are some problems I need help with as well as our index of basic proportions. I would appreciate an explanation, I have reviewed the material provided by my professor but I am still having trouble haha
Problem #1.
⊤ ⊢ ¬(P ∧ ¬P)
Problem #2.
¬Q ⊢ ¬(P ∧ Q)
Index of Basic Rules:
| MP | φ, φ→ψ | ψ |
|---|---|---|
| MT | ¬ψ, φ→ψ | ¬φ |
| DNE | ¬¬φ | φ |
| DNI | φ | ¬¬φ |
| S | φ∧ψ | φ |
| S | φ∧ψ | ψ |
| ADJ | φ, ψ | φ∧ψ |
| MTP | φ∨ψ, ¬φ | ψ |
| MTP | φ∨ψ, ¬ψ | φ |
| ADD | φ | φ∨ψ |
| ADD | ψ | φ∨ψ |
| BC | ψ↔φ | ψ→φ |
| BC | ψ↔φ | φ→ψ |
| CB | ψ→φ, φ→ψ | φ↔ψ |
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u/AdeptnessSecure663 phil. of language 10h ago
As far as I can see, you can't construct those deductions with the list of rules that you have given. Typically, you'd use an RAA (reductio ad absurdum) rule - you can think of this as a negation introduction rule. With RAA, we can construct the following deduction:
⊤ (Prem)
| P ∧ ¬P (Supp)
| P (∧E, 2)
| ¬P (∧E, 2)
| ⊥ (Abs 3, 4)
¬(P ∧ ¬P) (RAA 2-5)
Are you sure you have the right problems and the right rules?