Dear Bud, et al,
I have been on vacation hence this belated response to comments. In my humble opinion, I think it is wrong to claim that whatever cannot be observed directly is superfluous. Observation being of two varieties, one those things that can be physically observed by the senses, in the human situation possibly enhanced by instruments and those things that can only be apprehended by intuition and found to be in conformity with known physical law. Matter and waves are of the first variety, while Gravity, Action-at-a-distance, forces, etc are also observations that are real and cannot be discarded by science and categorized as "zombie ideas" or only "argued into existence philosophically". Matter cannot be used as the only definition of what substance is. Substance is anything that can act and can be acted upon. Substance is anything that can move. That is the point.
In apprehending the substantial nature of Space we must be broad in our definition of what "moves" means, and what "acted upon or can act mean".
In Bud, calling Einstein's statement, "...what is essential is merely that besides observable objects, another thing, which is not perceptible, must be looked upon as real...", problematic and a mistake, it will be useful to understand the basis for the statement, which is principally a recap of Newton's own conclusion from his famous globe thought experiment, which description I simplify here:
Imagine two globes at a fixed distance apart, seen by an observer to be rotating about their common centre of mass. Relative to each other there is no absolute or relative motion, yet a force exists between them related to their motion and calculated by mrω2. If the observer were to vanish will this force also vanish? If not, during the time the observer has vanished and before reappearing again, relative to what were the globes moving at angular velocity ω? And if you say, the force vanishes when the observer vanishes then would the moon also no longer orbit the earth if there is no observer? I don't think you will take that route.
Mach chose the non-vanishing reference during that period of observer disappearance as the fixed stars, while Newton chose a Space that has properties.
Other grounds to prove the existential nature of space stem from the behaviour and properties of Light. Light is a transverse wave, and a wave must be waving in something that can move or vibrate, if not it cannot be propagated. In spite of the unresolved theoretical difficulties, Einstein pointed this out in this lecture. More on this if requested.
Even within General relativity, Space when married with time in an ad hoc union called 'Space-Time' exists and is said to vibrate as gravitational waves. Space-Time can act on matter and can be acted upon by matter, in which case it is substantial. However, how a union can be substance and Space a component of the union is denied a substantial nature by modern day Einstein followers is what I think is corrected by this 1920 Einstein lecture at Leiden.
The term, "aether" has fallen into disrepute and any mention of it provokes an instantaneous rejection. Tom doesn't like it, neither does JRC and Steve Agnew. I would refrain therefore from using it. But I can discuss more the idea that Space is substance as Einstein suggested in this lecture.
Regards,
Akinbo