Hope I don't bore bejesus out of you here :)
Anyway, here's the next part..

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“Yes Roland, you’re so right, there was a trick to my explanation.” He smiled.

"I only addressed uniform motion. That's because I introduced a equivalence between all uniformly moving objects, making it impossible to decide any singular velocity inside it, without windows to see out from. Or even to differ standing still from free falling. When accelerating you will always know who is moving, except possibly when it's a uniform acceleration, being at a constant rate, creating the same feeling of heaviness that Earth does. But if we changed the reference from Earth to another mirror-set instead then the question as you so rightly concluded would have to be. ‘Which of those two pairs should age slower?’ Think of it, if my equivalence of all uniform motions expressing themselves the same are correct, then no matter which pair you were sitting on, the other mirror sets light-corn would seem to move slower. “

He stopped talking for a moment thinking.

”In a uniform motion the only way to decide who is moving is to use some sort of reference frame Roland. That also means that you have a free choice in defining which of those mirror-pairs that really are moving. Also, as I see it, making it possible to define both pairs as moving away from each other. This is somewhat of a problem and one of the ways the ancients used to solve it was to find another way of defining motion. The idea they at last thought describing the difference was to compare all motion against the stars. Those being so far away that they called them fixed, unmoving. It wasn’t the perfect solution perhaps but it worked for all practical reasons.”

Shadow couldn’t help feeling impressed by Roland’s deductive powers as he watched him mull over the new ideas. “So yes, you was right.” He concluded. “Those mirrors were also described as ‘frames of reference’ by the ancients, meaning that each pair presented you with their own unique mode of experience and observation, all depending on where you stood looking, your frame of reference as it was. And what you really questioned Roland, well, that was SpaceTime and how we could differ between motion and no motion. The ancients used those frames of reference to decide that, also defining some as inertial frames, meaning places without apparent motion relative what you compared it too.”

He looked at Roland to see if he still was listening and watching how he leaned forward, eager to not miss his slightest word, he nodded satisfied. “But those inertial frames are slippery subjects, let it suffice that all objects without acceleration, like our Earth, can be defined as an inertial frame as long as you compare to something moving uniformly relative it, keeping the same pace. Although, there is another way of defining if you’re in an inertial system. It’s called the equivalence principle, stating that all forms of gravitation is equivalent to acceleration.”

“How?”

“Simply because gravity accelerates, look at a apple falling and you will see its speed increase, it will hit you much harder falling from ten meters than from one, right? And to really understand the equivalence principle you also need to understand how the ancients thought about gravity, not as some force like a that steam locomotive pulling you with it, but more like invincible dips and bends made in space by all objects of mass, like our Earth, twisting space so that the path of less resistance always points to where the matter center is.”

“And why is acceleration so important?” asked Roland, starting to wish he had had a little less curiosity and a little more common sense, His headache now reaching cosmic proportions. “You can’t mean that gravity is acceleration?”

“Well, gravity is equivalent to acceleration in that if you accelerate you will feel heavier, and that added weight makes it inseparable from gravity. Acceleration can do two things, either reveal that you are in motion, or, in the case of a uniform constant acceleration, become indistinguishable from gravity itself. If it walks like a fish, swims like a fish and sings like a fish? What is it?”

“A whale?” Suggested Roland weakly.

Shadow tried to hide his smile as he watched Roland. “Possibly, but if it lays eggs then? Never mind Roland, in that motto our Earth too becomes a non-inertial object as its gravity can be seen as an form of uniform acceleration, but I warned you about that, didn’t I? Inertial frames are slippery subjects. But above all a inertial frame is defined by its relation to something else, stationary relative it. Take those two mirrors for example, bouncing that light corn between them. Each one is an example of an inertial frame relative the other one, as they both always stays at the same distance versus each other. And when you sit on one, you too have become a part of that inertial frame, as you now are stationary relative those mirrors. “

He smiled watching Roland’s unspoken disgust. “Don’t give up on me yet Roland, Have some patience. Motion without acceleration won’t tell you a thing about any speed, will it? As long as it follows that path of less resistance I mean. Our Earth moves for example, but as it’s not accelerating, and as it follows that simple path of less resistance we don’t feel any speed, do we? “

“But we can see it moving.” Protested Roland. “I only need to look at the sun to know that we’re moving.”

“Do you know how long it took the ancients to accept the idea that it was Earth revolving around the Sun, and not the other way around?” Shadow asked bemused. “Centuries my friend, centuries of experiments. Most expected the universe to revolve around us. We being the crown of creation after all, and so also the natural center of the universe.”

He sat there losing himself to the dancing flames, contemplating those years of unending search reaching the point in space and time where he and Roland was now, wondering if it had any relevance at all in the end. Sighing he looked up. “In any case, if it makes it any easier you could consider yourself and the mirrors enclosed inside a black box, making it impossible for you to compare your motion to anything outside it.” He took a sip from his tea, enjoying its flavor. “Ah well, uniform motion they called it, remember? Meaning that no change ever could be discernible, at least not with you being inside that black box. But we were discussing inertial systems, weren’t we?”

Watching Roland hesitantly nod he happily continued, warming up to his subject. “Well, any uniformly moving object can be defined as an inertial system, relative any object stationary to it, as long as they walk that path of less resistance. Even though I freely admit it as seeming slightly inconsequential, as all objects you compare against will be of matter, and so have a mass containing gravity. Still, defining it so was a big step from the ideas before, where we expected the whole universe to revolve around an unmoving Earth, presuming Earth the hub of the universe. In a way the Ancients kept that thought though, with the addition of any inertial frame just being an arbitrary definition, applicable to any uniformly moving system, meaning any system making us unable to define motion to while enclosed in that black box. Can you see their thoughts Roland?”

“Maybe?” answered Roland hesitatingly “Anything that moves without acceleration is impossible to define as moving, unless you have something else to compare it too, am I right? And even when comparing you can’t swear to it being you moving, it’s as right to say that the Earth is standing still for example, although all other paths then would be hopelessly convoluted.” He stopped for a second thinking over his own words. “Doesn’t that presume that there have to be some principle of simplicity involved Shadow?” He asked looking at his friend, his confusion growing. “There has to be, or we have nothing, even if I accepted those definitions?” He stopped feeling as if he was if he was standing in a quagmire, seeing all he ever had considered reason inexorably move away.

“Yes, there is a principle of simplicity. They referred to it as Occam's Razor, don’t ask me why. What it said was that of two competing theories the simplest one was to be preferred. And when it comes to whether it’s the Sun or the Earth that is our solar-systems center, then the Sun made the simpler choice.”