A note on existential instantiation

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Abstract
The presence of a rule for existential instantiation (EI) in a system of natural deduction often causes some difficulties, in particular, when it comes to formulate necessary restrictions on the rule for universal generalization (UG). A system containing rules for EI and UG that avoided Quine's rather cumbersome restrictions on these rules was formulated by Copi [2], but the system was found to be inconveniently restrictive. A less restrictive system was therefore suggested by Copi [3]. Also that system forces some deductions to be unnecessarily long as is shown in Prawitz [5, Appendix C, p. 104], where a way to liberalize Copi's restriction on UG is suggested (p. 105). However, the system suggested by Copi [3] is also incorrect (i.e. unsound) as has recently been shown by Parry [4] in this Journal.