What is Logic?

• Engineer, Mathematician, Logician joke about brown sheep (not Indian cricket team!) in New Zealand.
• What is the Indic word for logic?

For our current purposes, we can live with

Logic is a formal language with precise syntaxand semantics.

Issues

• Expressive Power (Propositional, First Order, Higher Order, Modal, ..)
• Decidability and Complexity
  • Sound (What I prove is correct)
  • Completeness (If something is correct, I can prove it)