## 辅导案例-PHIL1012

• July 19, 2020

PHIL1012 Introductory Logic Take-Home Exam Winter Main and July Intensive 2020 Instructions:​ Answer all parts of all questions. Read each question carefully. The mark value of each question is shown below, and 118 marks are available. The examination as a whole is worth 50% of your final mark for the course. Please clearly label your answers, so that it is always clear exactly which question you are answering at any given point in your submission. Questions: 1. Translate the following into GPLI: [32 marks: 4 marks per part] (i) Smith will start his engine if Jones is OK with it. (ii) Only horses gallop. (iii) Aristotle isn’t a rationalist. (iv) Neither Miriam nor Julie are sitting between Fisher Library and The Quad. (v) Not everyone Scott knows is a student of Miriam. (vi) Julius can see everything anyone writes. (vii) Everyone except Miriam is on Facebook. (viii) I know exactly one person who has been to Chile. 2. Here is a model: Domain: {2, 7, 11, 10} Referents: ​a​: 7 ​c​: 2 ​g​: 10 ​h​: 2 Extensions: N: ​{2, 7} F​: Ø Q​: {7, 10} L​: {11} T​: {⟨11, 11⟩, ⟨11, 11⟩, ⟨11, 11⟩, ⟨11, 11⟩} M​: {⟨11, 11⟩, ⟨7, 11⟩, ⟨10, 2⟩} P​: {⟨7, 2, 10⟩} R​: {⟨7, 7, 10, 7⟩} For each proposition below, say whether it is true or false on the above model. Explain your answers with reference to the semantics (i.e. the truth rules) of the relevant operators. 1 PHIL1012 Introductory Logic – July Intensive – Take-Home Exam [30 marks: 5 marks per part] (i) (​Qa ​→ ​Mca​) ∧ ㄱ​Lc (ii) ∀​y​(ㄱ​Qy​ → ​Ly​) (iii) ∀​x​(​x ​= ​a ​∨ ​x ​= ​c ​∨ ​x ​= ​g ​∨ ​x ​= ​h​) (iv) ∃​y​(​Ly ​∧ ∃​zMzy​) (v) ​Pacg ​→ ∀​x​(​Fx ​→ ∃​y​(​Myh ​∧ ㄱㄱㄱ​Ryaba​)) (vi) ∃​x​∃​y​∃​z​(∃​wMwx ​∧ ∃​wMwy ​∧ ∃​wMwz ​∧ ​y​ ≠ z) 3. Use the tree method to answer the following questions. Justify your answers. [30 marks: 6 marks per part] (i) Is the following set of propositions satisfiable? If so, read off a model on which they are all true. {​Rab​, ∃​x​(​Rxb ​∧ ​x​ = ​c​), ㄱ​c ​≠ ​a​} (ii) Are the following propositions equivalent? If they are not, read off a model on which their truth values differ. a ​= ​b ​∧ ㄱ∀​xRabx b ​= ​a ​∧ ∃​y​ㄱ​Rbay (iii) Is the following proposition logically true? If it is not, read off a model on which it is false. ∀​x​(​Px ​→ ​Qx​) ∧ ∃​y​(​Py ​↔ ​Py​) (iv) Is the following argument valid? If it is not, read off a model on which its premises are true and its conclusion is false. ∃​x​∀​y​(​Py ​→ ​Ryx​) ∀​x​(​Sax ​→ ​Px​) Sab ∴ ∃​xRbx (v) Is the following argument valid? If it is not, read off a model on which its premises are true and its conclusion is false. 2 PHIL1012 Introductory Logic – July Intensive – Take-Home Exam ∃​x​(​Bx ​∧ ㄱ​Sxa​) ∀​x​(​Bx ​→ ​x ​= ​a​) ∴ ㄱ​Saa 4. Questions involving a new concept: [10 marks: 2 marks per part] Let us say that a formula of ​GPLI​ is ​fickle​ if it is possible to take a model on which it is true, and add objects to the domain (changing nothing else in the model) so that the formula is false on the resulting model. By ‘changing nothing else’, I mean that these extra objects must not be given names or put into the extensions of any predicates, and that existing referents and extensions on the model remain unchanged. To explain this idea of a fickle formula in other words: Say that a model M′ of GPLI is an outgrowth ​of a model M of GPLI iff: – M′ and M assign the same referents to the same names and the same extensions to the same predicates. – Every object in the domain of M is in the domain of M′. Now, a formula of GPLI is ​fickle ​iff there is a model M on which it is true ​and​ there is an outgrowth of M on which is it false. For each of the following formulas, say whether or not it is fickle and justify your answer: (i) ∀​x​(​x ​= ​a ​∨ ​x ​= ​b ​∨ ​x ​= ​c​) (ii) ∀​xFx (iii) ∀​x​(​Fx​ → ​Gx​) (iv) ∃​z​(​Lz ​∧ ㄱ​Lz​) (v) ∃​x​∃​yx​ ≠ ​y 5. Short answer questions: [16 marks: 4 marks per part] 3 PHIL1012 Introductory Logic – July Intensive – Take-Home Exam (i) In logic, we translate ‘All humans are mortal’ (and similar propositions) in a particular way. The way we translate it, it could be true even if there are no humans. With reference to the semantics of MPL (or GPL, or GPLI), explain how it is possible for the translation to be true even if there are no humans. (ii) Give, in English, an example of an argument which is necessarily truth-preserving but not valid, and explain why it is not valid. (iii) Give, in English, a valid argument whose validity cannot be captured in MPL but can be captured in GPL. Explain why its validity cannot be captured in MPL. (iv) What is wrong with the expression ‘This proposition is satisfiable on all models’? (For the purposes of the question, it doesn’t matter what the proposition is.) 4

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