New posts in First Order Logic

Is it legitimate to quantify a variable twice?

Defining ordered pairs using only first-order logic

Is the "ancestor" relationship impossible to define in first-order logic?

What exactly is a contradiction and how does it differ from falsity?

Motivating the Implication Operator

What kinds of theories of real multiplication with one real constant are possible?

Why doesn't universality imply existence?

What is true about a proof system that is complete but not sound?

Everyone loves all lovers, Romeo loves Juliet $\vdash$ I love you (Natural deduction proof)

Definability in the signature of first-order logic with identity