A reader who reports that his main interest is in contemporary metaphysics inquires:
Should I learn as much logic as humanly possible during my PhD? Or should I learn only what I need along the way? I have a basic grasp of symbolic and predicate logic, but little meta-logic.
First of all, it makes no sense to oppose symbolic to predicate logic. Modern symbolic logic includes both propositional logic and predicate logic.
Second, learn what you need as you go along. For example, existence is one of the central topics in metaphysica generalis. To work on this topic in an informed way you have to understand the modern quantificational treatment of existence in mathematical logic.
Here is the minimum required for doing metaphysics. First, a thorough grounding in traditional formal logic including the Aristotelian syllogistic. Second, modern symbolic logic including the propositional calculus and first-order predicate logic with identity. Third, some familiarity with axiomatics and the concepts of metalogic including consistency and completeness of axiom systems. Fourth, axiomatic set theory. Fifth, some (alethic) modal logic both propositional and quantified.
The best way to master these subjects, or at least the first two mentioned, is by teaching them to undergraduates.
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