es that are located in space
&gt; &gt; and time: abstract things like sets and numbers are not classified in
&gt; &gt; either of these ways. (Note in particular that a set of physical
&gt; &gt; things is not itself physical.) The basic test is to ask whether the
&gt; &gt; thing in question is best thought of as something that can be said to
&gt; &gt; be 'happening' or whether one thinks of it as something that endures
&gt; &gt; for a time and participates in happenings. As an alternative
&gt; &gt; criterion