t proper.
 
JA: Nevertheless, we do use mathematical systems to describe
    time-evolving physical systems.  And when we do this, it
    is perfectly possible for a function f : R -&gt; X to give,
    for every real time associated with a real number t in R,
    the position of the test object in a physical space that
    is parameterized by a mathematical space X.  In this way,
    then, in the only figure of speech that could make sense,
    the &quot;associate membership&quot; of the test object in vari