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 pa

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. Plank'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.