The primitive symbols of E are those of F, plus the symbol ∃. The formation Rules of E are those of F, plus the following If B is a wff of E and x is an individual variable, then ∃xB is a wff of E. The axiom schemata of E are those of F plus
Interpretation over a singleton Let I be , and σ ∈ ΣI. 1. I(A)(σ) = I(∀xA)(σ). 2. I(t)(σ) = a. 3. I(Sx1,···,xn t1,···,tn A)(σ) = I(A)(σ). 4. I0(P), σ(P) ∈ {I(n), Ψ(n)} for every n-ary predicate constant (variable), where
F= = F + “ = ” + 2 Axiom Schemata Axiom Schema 6 x = x. Axiom Schema 7 x = y ⊃ (SzxA ⊃ SzyA) where A is an atomic wff. A first order theory is a first-order theory with equality if it has a binary predicate = such that the wffs above are theorem of the theory