I'm majoring in mathematics, and have a deep interest in logic (-related) fields.
I have studied Robert Causey's Logic, Sets And Recursion, so I think I have had a basic knowledge of sentential calculus and predicate calculus.
Now I am very interested in topics on answering the questions such like
- "what axioms truly are?"
- "what does a definition truly mean?"
- "what is the the definition of equality $=$"
- what is the basis of all mathematics?"
- "what is a formal system?"
These question seem to fall in many fields of mathematics, such as mathematical logic, metamathematics, type theory, model theory, category theory. (I'm not sure whether I'm correct.)
So if I want to learn this and get a complete understanding of these, which books would you recommend to me to read next, and in what order? (I'm afraid that I accidentally read the advanced books without having read the basic books yet.)
And I'm curious about the differences between type theory, modal theory and metamathematics; maybe I don't need to study these all to answer my curious questions stated above.
I've looked at brief introductions of these subject in the wikipedia, but still don't understand it very well.
Is it good to read Introduction to Metamathematics by Kleene?