 It remains to explain what it is for a method m of determining the extension of t to rest upon an axiom \(\phi\). This relation obtains if and only if the use of m depends on \(\phi\) being a true sentence. In other words, m rests upon \(\phi\) if and only if the hypothetical assumption of \(\phi\) being false or indeterminate would invalidate the use of m in the sense that we would be lacking the commonly presumed justification for using m. The qualification ‘or some part of that extension’ has been introduced in the present definition because we cannot expect a single measurement method to determine the extension of a scientific quantity completely. T-non-theoreticity is the negation of T-theoreticity: