I think this is pretty clean cut,
Even in our world through advancements in knowledge of theoretical math we are able to predict things before finding them "Because it works like this then the outcome should be this" granted we are not always right, Both Aizen and Urahara being the smartest intellects of The Gotei 13 which is a group of the 2nd best collection of the strongest and most intelligent souls of all history that did not become hollows, with intellect like that they would most likely to be able to deduce the workings of their natural worlds as a most basic of things, and both have seen something that no one else has through this superior knowledge.
Given that Mayuri knows the science girl of the Royal Guard, we must assume shes gone to visit him, either when he was in jail, or the vice captain of Urahara, which the latter is much more likely given Urahara has access to information to the workings of the Royal Guard, enough to have had a sample to analyze.
As far as Aizen goes, once absorbing the hogyoku and becoming a being of a "higher dimension" he was able to see past what normal souls see. (His conversation with Urahara was when he was beyond all his transformations)
However if this is true, and Ichigo truly did surpass him, then Ichigo has seen this as well.
Speculation! DI DU DUN!
^ I think you're in the minority. Aizen is popular, because he's such an epic villain - he managed to fuck up the whole of SS despite the odds, and only got beaten because he wasn't the main char . More of him is more of that stuff. Plus, he has a connection to the recently presented Soul King, and along with his appearance, we could find out more about that. His role in the story is far from over.
I have a number of theories on this;
The first, and the most obvious IMO, is that Urahara knew about, saw, or even met the Soul King through his connections with Yoruichi, who in turn most likely has connections with the Soul King through her being royalty. So how did Aizen come to see the Soul King? Well, Urahara's words suggested that Aizen didn't actually meet the Soul King in person, but instead somehow got a glimpse of him through covert methods. My best guess is that Aizen was sneaking around spying on Urahara (which there's been a lot of evidence of him doing throughout the entire Bleach timeline!) and managed to see the Royal Dimension / Soul King as a result of this (through bugs planted on Urahara or something similar).
My second guess is that it had something to do with the Hougyoku; one theory is that it allowed both Urahara and Aizen to detect the Soul King by increasing their power levels and thus their awareness (going back to the 2D beings can't interact with 3D beings). Aizen may have not literally seen the Soul King, but instead detected his Reiatsu; enough to let him make up his mind about overthrowing him...which kinda makes sense...
...So imagine you're a bright-spark Shinigami (yes, awesome, I know) called Aizen, and you've been happily been following the rule of your beloved Shinigami King...sure, you've never seen him, and his power is too great for you to even detect him, but who cares eh? Now, being a dabbler, you create this super sphere-shaped gizmo which you've designed to soup up your beloved Shinigami King's army. Oh, look at that, it gives you a boost in power...enough to detect your beloved Shinigami Ki-...wait...WTF?!?! Why the hell does our SHINIGAMI King reek of Quincy?!?!?! Yes, your universe has just been ruined!!!!!
Their discussion implies Aizen actually see him, so "sensing" him is not really an option. Why can't Ichigo sense him? He has bigger reiatsu than Urahara, possibly bigger than Aizen. Hogyoku or not, Ichigo would have to sense SK. And tht thing can't possibly be a Quincy. It's a damn mix of every damn thing. It's a Hollow-like entity that can manipulate souls and control it and is Shinigami King. Hollow/Quincy/Shinigami, hello Ichigo. And that's exactly why it is SOUL king, not SHINIGAMI king.
Maybe they both had the bright idea of planting monitoring devices on Hikifune before she went up? Knowledge on the RG and how they are recruited is likely restricted to only a select few, but they are both the kind of guys to delve into knowledge they aren't supposed to. Urahara might have been told she was being promoted to RG when he was handed his own promotion, and taken the opportunity to plant something on her. Aizen didn't seem to know about the RG until Shunsui told him, but that may well have been part of his act. Maybe he also put a bug on her when he found out she'd be going to the Royal Realm, since he was planning on going there himself one day after all.
In Aizen's case, it's possible he knew about the RG and the method of their induction. If he was already planning on slaying the Soul King, he must have done some research into his defences. It wouldn't surprise me if he was just acting when he asked Shunsui about the RG, but of course it's possible he genuinely didn't know, as he was only a VC at the time and probably not granted the kind of access to information a Captain would be.
Hikifune got promoted, she went up, 2 (?) days after she went Yoruichi recommented Urahara for her seat. I don't think he knew about her promotion and made all needed arrangements to spy on her and SK. And he already had his inventions back then.
Last edited by Duniak; August 25, 2013 at 07:15 AM.