I have been trying to study first-order logic to have a sound basis on mathematical language. The main target is to have a clear path: I start with first-order logic (the language), then I go and study set theory, which is in fact a series of axioms (ie, a series of statements of the language), and then, from set theory, the whole mathematics can be deduced from that. However, I have found a problem I did not expect to find. When I read set theory books, they mostly do not state clearly the laws of the first-order logic (or they do it in a too simple way; but it is OK, they are books on set theory, not on logic). But I was expecting that when I started reading books on first-order logic, these books would be only about logic and the language. However, to my surprise all books I have been checking up to now resort to the concept of sets (in an intuitive way, they do not define sets most of the time). I even have read a comment on that in a pdf I have found in the web: A Problem Course in Mathematical Logic Version 1.5 Volume I Propositional and First-Order Logic Stefan Bilaniuk where it states in the Appendix A, devoted to Set theory: "The properly sceptical reader will note that setting up propositional or first-order logic formally requires that we have some set theory in hand, but formalizing set theory itself requires one to have first-order logic." So, it is not only my impression. Is there some good book that studies first-order logic without resorting to set theory or any other more advanced mathematics, such that it can be used as the foundations to study set theory on a second step?