We know what space is, at least operationally.  Or rather space-time.  Space is what can be measured with rigid rods.  Time is what clocks measure.  Einstein painstakingly defined those concepts this way.<p>Plank&#x27;s question is more metaphysical : beyond what it means experimentally, does space have a -physical- existence per se, even in a completely empty universe?  In other words, is an empty universe different than no universe at all?  If not, one way to look at space it is to consider it as a set of abstract rules regarding the possible interactions between particles, those rules being very close to what we call geometry.