Suppose that we have a measuring tape available to measure distance, and suppose further that our only method to measure weight is the use of beam balance scales.
let us denote by z the entity consisting of the balanced beam scale, the child, and the counterbalancing measuring weight; then the validity of our measuring result depends on a statement z is an AS in order to check statement (2), we have to presuppose statement (3), which is of the very same form and uses the very same predicate.