Language, Proof and Logic

Errata

Page 4: The last sentence of the last full paragraph should read "If our economic theory says that inflation is a necessary consequence of low unemployment, but today's low unemployment has not caused inflation, then our economic theory needs reassessment."
Page 120: Line 7 (Commutativity of disjunction). The second word should be disjunction.
Page 186: Exercise 7.11. The Show/Hide Labels menu item is on the World menu, not on the Display menu.
Pages 292-3: The conclusion of the arguments presented in steps 1 and 2 of the You Try It are not the same as those in the files described (the conjuncts are reordered). The formula immediately after the argument in step two of the You Try It is also incorrect. It should be the goal formula from the file Lemma 1. The You Try It exercise should be completed using the exercise files from the disk.
Page 293: You Try It. Instruction 3 should say "...the same number of formulas are cited as there are premises in the lemma file...".
Page 293: You Try It. Instruction 4 should say "...application of ∨-Intro.".
Page 448, Axiom of Replacement. The description of the Axiom of Replacement here is incorrect. Rather than concerning functions, the axiom schema asserts the existence of a set { y | ∃x (x ∈ a ∧ P(x,y)) }, provided that the formula P(x,y) holds of a exactly one y for each x in a set a, i.e. ∀x (x ∈ a → ∃!y P(x,y)).
Page 448, Footnote 4: should read "Technically speaking, it is a consequence ..."

The file, Taut Con 1, which is used in the You Try It on page 114, contains an error. Step 3 contains the incorrect formula.