?

Existence is then
identified as that which is expressed by the existential quantifier of the predicate
calculus, as for example in Quine's famous `criterion of ontological commitment',
according to which to be is to be the value of a bound variable in a true first-order
formalization of a natural scientific theory (Quine, 1953).

I think this might be the most complicated sentence I've read in my life. Does anyone know what it means?