More and more, he began to think in terms of a room whose floor was the absolute truth about the universe: where it had come from, how it worked, what it meant to be a thinking being in that universe. But that floor looked very much like a carpet, and it was in turn concealed by other carpets, one on top of the other, each of which represented some imperfect approximation to the final layer. Each layer might look convincing, might endure decades or centuries of enquiry without hinting that it contained a flaw, but sooner or later one would inevitably reveal itself. A tiny, loose thread - perhaps a discrepancy between observation and theory - and with a tug the entire fabric of that layer would come apart. It was in the nature of such revolutions that the next layer down would already have been glimpsed by then. Only the final carpet, the floor, would contain no logical inconsistencies, no threads waiting to be unravelled.
Could you ever know when you'd reached it? John wondered. Some thinkers considered it impossible to ever know with certainty. All you could do was keep testing, tugging at every strand to see how firmly it was woven into the whole. If after tens of thousands of years the pattern was still intact, then it might begin to seem likely that you had arrived at final wisdom. But you could never know for sure. The ten thousand and first year might bring forth some trifling observation that, as innocent as it first seemed, would eventually prove that there was yet another layer lurking underneath.
You could go on like that for ever, never knowing for sure.
Or - as some other thinkers speculated - the final theory might come with its own guarantee of authenticity, a golden strand of logical validation threaded into the very mathematical language in which it was couched. It might be in the nature of the theory to state that there could be no deeper description of the universe.
But even then, it wouldn't stop you making observations. It wouldn't stop you testing.
John kept learning. M-theory became a distant and trifling obstacle, dwarfed by the daunting unified theories that had superseded it. These theories probed the interface not just of matter and space-time, but also of consciousness and entropy, information, complexity and the growth of replicating structures. On the face of it, they seemed to describe everything that conceivably mattered about the universe.
But each in turn was revealed as flawed, incomplete, at odds with observation. An error in the predicted mass of the electron, in the twenty-second decimal place. A one-in-ten-thousandth-part discrepancy in the predicted bending of starlight around a certain class of rotating black hole. A niggling mismatch between the predicted and observed properties of inertia in highly charged space-time.
The room contained many carpets, and John had the dizzying sense that there were still many layers between him and the floor. He'd made progress, certainly, but it had only sharpened his sense of how far he had to go.
The Kind remade him time and again, resetting his body clock to give him the time he needed for his studies. But each leap of understanding pushed him closer to the fundamental limits of a wet human brain wired together from a few hundred billion neurons, crammed into a tiny cage of bone.
'You can stop now, if you like,' the Kind said, in the hundredth year of his quest.
'Or what?' John asked mildly.
'Or we continue, with certain modifications.'
John gave them his consent. It would mean not being human for a little while, but given the distance he had come, the price did not strike him as unreasonable.
The Kind encoded the existing patterns of his mind into a body much like one of their own. For John, the transition to a machine-based substrate of thinking crystal was in no way traumatic, especially as the Kind assured him that the process was completely reversible. Freed of the constraints of flesh and scale, his progress accelerated even more. From this new perspective, his old human mind looked like something seen through the wrong end of a telescope. Compared to the mental mansion he now inhabited, his former residence looked as squalid and limiting as a rabbit hutch. It was a wonder he had understood anything.
But John wasn't finished.
A thousand years passed. Always adding new capacity to himself, he had become a kilometre-high crystalline mound on the summit of Pavonis Mons. He was larger by far than any of the Kind, but that was only to be expected: he was probing layers of reality that they had long since mapped to their own satisfaction, and from which they had dutifully retreated. Having attained that understanding once, the Kind had no further need for it.
There were other people on Mars now. John had finally acquiesced to the Kind's offer to bring him companions, and they had created children who had now grown to become parents and grandparents. But when John agreed to the coming of other humans, it had little to do with his own need for companionship. He felt too remote from other humans now, and it was only because he sensed that the Kind wished to perform this exercise - that it would please the aliens to have something else to do - that he had relented. But even if he could not relate to the teeming newcomers, he found it pleasing to divert a small portion of his energies to their amusement. He rearranged his outer architecture - dedicated to only the most trivial data-handling tasks - so that he resembled an ornate crystalline fairy palace, with spires and domes and battlements, and at dusk he twinkled with refracted sunlight, throwing coloured glories across the great plains of the Tharsis Montes. A yellow road spiralled around his foot slopes. He became a site of pilgrimage, and he sang to the pilgrims as they toiled up and down the spiral road.
Millennia passed. Still John's mind burrowed deeper.
He reported to the Kind that he had passed through eighteen paradigmatic layers of reality, each of which had demanded a concomitant upgrade in his neural wiring before he could be said to have understood the theory in all its implications, and therefore recognised the flaw that led to the next layer down.
The Kind informed him that - in all the history that was known to them - fewer than five hundred other sentient beings had attained John's present level of understanding.
Still John kept going, aware that in all significant respects he had now exceeded the intellectual capacity of the Kind. They were there to assist him, to guide him through his transformations, but they had only a dim conception of what it now felt like to be John. According to their data less than a hundred individuals, from a hundred different cultures, all of them now extinct, had reached this point.
Ahead, the Kind warned, were treacherous waters.
John's architectural transformations soon began to place an intolerable strain on the fragile geology of Pavonis Mons. Rather than reinforce the ancient volcano to support his increasing size and mass, John chose to detach himself from the surface entirely. For twenty-six thousand years he floated in the thickening Martian atmosphere, supported by batteries of antigravity generators. For much of that time it pleased him to manifest in the form of a Bosendorfer grand piano, a shape reconstructed from his oldest human memories. He drifted over the landscape, solitary as a cloud, and occasionally he played slow tunes that fell from the sky like thunder.
Yet soon there came a time when he was too large even for the atmosphere. The heat dissipation from his mental processes was starting to have an adverse effect on the global climate.
It was time to leave.
In space he grew prolifically for fifteen million years. Hot blue stars formed, lived and died while he gnawed away at the edges of certain intractables. Human civilisations buzzed around him like flies. Amongst them, he knew, were individuals who were engaged in something like the same quest for understanding. He wished them well, but he had a head start none of them had a hope of ever overtaking. Over the years his density had increased, until he was now composed mostly of solid nuclear matter. Then he had evolved to substrates of pure quark matter. By then, his own gravity had become immense, and the Kind reinforced him with mighty spars of exotic matter, pilfered from the disused wormhole transit system of some long-vanished culture. A binary pulsar was harnessed to power him; titanic clockwork
enslaved for the purposes of pure mentation.
And still deeper John tunnelled.
'I . . . sense something,' he told the Kind one day.
They asked him what, fearing his answer.
'Something ahead of me,' he said. 'A few layers down. I can't quite see it yet, but I'm pretty sure I can sense it.'
They asked him what it was like.
'An ending,' John told them.
'This is what we always knew would come to pass,' the Kind told him.
They informed him that only seven other sentient beings had reached John's current state of enlightenment; none in the last three billion years. They also told him that to achieve enlightenment he would have to change again, become denser still, squeezed down into a thinking core that was only just capable of supporting itself against its own ferocious gravity.
'You'll be unstable,' they told him. 'Your very thought processes will tend to push you into your own critical radius.'
He knew what they meant, but he wanted to hear them spell it out. 'And when that happens?'
'You become a black hole. No force in the universe will be able to prevent your collapse. These are the treacherous waters we mentioned earlier.'
They said 'earlier' as if they meant 'earlier this afternoon', rather than 'earlier in the history of this universe'. But John had long since accustomed himself to the awesome timescales of the Kind.
'I still want you to do it. I've come too far to give up now.'
'As you wish.'
So they made him into a vast ring of hyperdense matter, poised on the edge of collapse. In his immense gravitational field, John's lightning thought processes grew sluggish. But his computational resources were now vast.
Many times he orbited the galaxy.
With each layer that he passed, he sensed the increasing presence of the ending, the final, rock-hard substrate of reality. He knew it was the floor, not another mirage-like illusion of finality. He was almost there now: his great quest was nearing its completion, and in a few thoughts - a few hours in the long afternoon of the universe - he would arrive.
Yet John called a halt to his thinking.
'Is there a problem?' the Kind asked, solicitously.
'I don't know. Maybe. I've been thinking about what you said before: how my own thought processes might push me over the edge.'
'Yes,' the Kind said.
'I'm wondering: what would that really mean?'
'It would mean death. There has been much debate on the matter, but the present state of understanding is that no useful information can ever emerge from a black hole.'
'You're right. That sounds an awful lot like death to me.'
'Then perhaps you will consider stopping now, while there is still time. You have at least glimpsed the final layer. Is that not enough for you? You've come further than you could ever have dreamed when you embarked on this quest.'
'That's true.'
'Well, then. Let this be an end to it. Dwell not on what is left to be done, but on what you have already achieved.'
'I'd like to. But there's this nagging little thing I can't stop thinking about.'
'Please. To think about anything in your present state is not without risk.'
'I know. But I think this might be important. Do you think it's coincidence that I've reached this point in my quest, at the same time as I'm teetering on the edge of collapsing into myself?'
'We confess we hadn't given the matter a great deal of thought, beyond the immediate practicalities.'
'Well, I have. And I've been thinking. Way back when, I read a theory about baby universes.'
'Continue . . .' the Kind said warily.
'How they might be born inside black holes, where the ordinary rules of space and time break down. The idea being that when the singularity inside a black hole forms, it actually buds off a whole new universe, with its own subtly altered laws of physics. That's where the information goes: down the pipe, into the baby universe. We see no evidence of this on the outside - the expansion's in a direction we can't point; it isn't as if the new universe is expanding into our own like an explosion - but that doesn't mean it isn't happening every time a black hole forms somewhere in our universe. In fact, it's entirely possible that our universe might well have been budded off from someone else's black hole.'
'We are aware of this speculation. And your point being?'
'Perhaps it isn't coincidence. Perhaps this is just the way it has to be. You cannot attain ultimate wisdom about the universe without reaching this point of gravitational collapse. And at the moment you do attain final understanding - when the last piece falls into place, when you finally glimpse that ultimate layer of reality - you slip over the edge, into irreversible collapse.'
'In other words, you die. As we warned.'
'But maybe not. After all, by that point you've become little more than pure information. What if you survive the transition through your own singularity, and slip through into the baby universe?'
'To become smeared out and reradiated as random noise, you mean?'
'Actually, I had something else in mind. Who's to say that you don't end up encoding yourself into the very structure of that new universe?'
'Who's to say that you do?'
'I admit it's speculative. But there is something rather beautiful and symmetrical about it, don't you think? In the universes where there is intelligent life, one or more sentient individuals will eventually ask the same questions I asked myself, and follow them through to this point of penultimate understanding. When they achieve enlightenment, they exceed the critical density and become baby universes in their own right. They become what they sought to understand.'
'You have no proof of this.'
'No, but I have one hell of a gut feeling. There is, of course, only one way to know for sure. At the moment of understanding, I'll know whether this happens or not.'
'And if it doesn't--?'
'I'll still have achieved my goal. I'll know that, even as I'm crushed out of existence. If, on the other hand, it does happen . . . then I won't be crushed at all. My consciousness will continue, on the other side, embedded in the fabric of space and time itself.' John paused, for something had occurred to him. 'I'll have become something very close to--'
'Don't say it, please,' the Kind interjected.
'All right, I won't. But you see now why I hesitate. This final step will take me as far from humanity as all the steps that have preceded it. It's not something I'm about to take lightly.'
'You shouldn't.'
'The others . . .' John began, before trailing off, aware of the fear and doubt in his voice. 'What did they do, when they got this far? Did they hesitate? Did they just storm on through?'
'Only three have preceded you, in all of recorded history. Two underwent gravitational collapse: we can show you the black holes they became, if you wish.'
'I'll pass. Tell me about the third.'
'The third chose a different path. He elected to split his consciousness into two streams, by dividing and reallocating portions of his architecture. One component continued with the quest for ultimate understanding, while the other retreated, assuming a less-dense embodiment that carried no risk of collapse.'
'What happened to the component that continued?'
'Again,' the Kind said, with the merest flicker of amusement at John's expense, 'we'd be delighted to show you the results.'
'And the other half? How could he have preserved the understanding he'd achieved, if he backtracked to a simpler architecture?'
'He couldn't. That's exactly the point.'
'I don't follow. Understanding required a certain level of complexity. He couldn't have retained that understanding if he stripped himself back.'
'He didn't. He did, however, retain the memory of having understood. That, for him, was sufficient.'
'Just the memory?'
'Precisely that. He'd glimpsed enlightenment. He didn't need to retain every detail of that glimpse to know
he'd seen it.'
'But that's not understanding,' John said exasperatedly. 'It's a crude approximation, like the postcard instead of the view.'
'Better than being crushed out of existence, though. The being under discussion seemed adequately content with the compromise.'
'And you think I will be too?'
'We think you should at least consider the possibility.'
'I will. But I'll need time to think about it.'
'How long?'