  A major disagreement between different views about the foundations of quantum
mechanics concerns whether for a theory to be intelligible as a fundamental
physical theory it must involve a "primitive ontology" (PO), i.e., variables
describing the distribution of matter in 4-dimensional space-time. In this
paper, we illustrate the value of having a PO. We do so by focussing on the
role that the PO plays for extracting predictions from a given theory and
discuss valid and invalid derivations of predictions. To this end, we
investigate a number of examples based on toy models built from the elements of
familiar interpretations of quantum theory.
