'I used to think everything was my fault,' Severus said at last. 'And then – when I died and came here, and I realised that a lot of what had happened back in my life hadn't been all my fault, but that I still couldn't be happy here, I decided nothing was my fault. If I couldn't be happy, it was because everyone was always going to hurt or reject or betray or exploit me just the way they always had, because nobody cared about me and nobody ever would. So if Cheiron wanted me to talk to someone about my problems, whether it was him or you or anyone else, I had to get away in case he just wanted to ensnare and manipulate me. So if I stayed a safe distance away and established myself as his potion-brewer, I could tell myself the only reason he wanted me here was because I was a useful slave, and I could resent him for it, and that was safer than letting myself trust him.'

He remembered, in one of the memories they had watched, Aral Vorkosigan telling Cordelia Naismith about Konstantine: 'He hates my guts. He enjoys hating my guts. It seems to be necessary for his ego, somehow.' But Konstantine, however much he might tell himself at a superficial level that he hated all Vor in case they were all like Ges Vorrutyer, had enough emotional wisdom to sense that Aral and Cordelia were people he could trust, people it was worth being loyal to. Severus had never had that instinct. Maybe he had been born with bits missing from his brain which were simply different from those missing from Konstantine's.

'And then when you turned up, it was the same. If I let myself think you could be my friend, you could reject me and hurt me, so I had to reject you first. I thought it wouldn't be so bad letting Konstantine, and later Anakin, come and live with me, because I assumed they weren't capable of loving me and so I could just be protective of them without expecting anything in return, as if they were my students. But it didn't work out like that.' Because they did care about him, but they couldn't stop him from isolating himself if he wanted to. No – not couldn't. Anakin was a Sith, after all – he was perfectly used to forcibly using Legilimency on people if he wanted to. But he wanted to be a better person, so he had deliberately restrained himself from forcing his way into Severus's mind once Severus had stopped consenting to link minds with him. And Konstantine wasn't comfortable with even preventing people he loved from physically harming themselves, if they were people he considered his betters, or if intervening risked hurting them. What was he supposed to do about a wizard shutting himself off from communication by turning into a cat?

'That's insane, isn't it?' he remarked. 'It's completely illogical.'

'It isn't logical in the sense that a mathematical equation is logical,' said Nutt. 'But it is logical in the sense that someone who is used to writing numbers in Latatian numerals might prefer to continue to use them even though the Klatchian number system is more efficient and flexible.'

'What's logical about that?'

'A human's brain is the most energy-hungry part of your body,' said Nutt. 'Thinking the thoughts that you are used to thinking, instead of finding new thoughts, is more energy-efficient, in the same way that it is easier to use a muscle that you have trained and built up, or easier to walk along a well-worn path than to cut a new one through dense undergrowth. Babies experience everything as new, and so they need to sleep for much of the time to allow their brains time to make sense of everything they have learned. But by the time you are an adult, you have already developed a well-honed set of mental patterns – whether in knowing how to brew potions and cast spells, or knowing not to trust people.'

'This is neural pruning, isn't it?' said Severus. 'If I'm an adult, I can bolt on new skills to what I already know, like learning a new potion recipe. But if I ever had the mental circuits for being happy or being likeable or having friends or making good decisions about which people to trust – if I wasn't just born with those bits missing so that I couldn't ever be normal – then they got pruned away when I was a teenager because I hadn't used them and so my brain decided I was never going to use them and there was no point in keeping them open. And even if I look like a teenager now, I must still have an adult brain if I can still remember being an adult, so I can't get back to actually being a child and being capable of learning unless I delete every memory of everything that happened to me. Is that true?'

'Not all of it,' said Nutt. 'Firstly, it isn't true that adults cannot learn and develop. Everyone's brain is constantly developing. Adults learn in a different way from babies, by seeing parallels with things they already know and understand, instead of learning everything from a standing start. This is why so much of language is metaphorical, like "seeing" or "standing start" or even "understand", which is such a well-worn metaphor that it is easy to forget that it even is a metaphor. And this process would already have been beginning from childhood. It isn't that a twenty-five-year-old suddenly loses the ability to learn new ideas when an eighteen-year-old, or even a twelve-year-old, would be able to. You have said that you cannot really be a teenager if you have an adult's memories in a thirteen-year-old body. But, thinking back to when you really were thirty-eight, and to when you really were thirteen, did you feel like a very different person at those ages?'

'No. I was just Severus.'

'Can you remember how you felt as a very young child? Were you the same person then?'

'I suppose – my first memory was lying in the bathtub, when I was a baby. Not having a bath – we had a tin bathtub, but we didn't have indoor running water, let alone hot water, so my mum used the bath as my crib, when I'd started crawling and couldn't just be left in a basket on the floor. Mum had made a spell to make it look as if bats were flapping around, just out of reach. They made me laugh.

'That's just one glimpse. But I've got a few memories from when I was two, and I think – I was happy, then. It was before I did my first accidental magic and dad found out that mum was a witch and he started hating us.

'I think I was always different. I didn't know how to play with other children when mum took me to the park to run around. And I was frightened by loud noises or strange smells, and I didn't want to be kissed or cuddled. I don't mean anyone was doing anything creepy to me, then – I just couldn't see the point in sitting on someone's lap when I wanted to run around and play. But I loved my dad, then, and I wasn't frightened of him at all.

'But then – it was 5th November, so I suppose I was two months short of three, and we'd gone to a firework party at the Scout hut for all the families in the area. And the first firework was loud and it scared me, and somehow all the other fireworks in the box turned into bats and flew away. And after that, dad understood that I was a wizard, and he hated me. Well, I suppose he was afraid of me, but I didn't know that. I just knew that I was afraid of him, and that when anything odd happened he'd beat me, and being hit, or being scared of being hit, made me make more things happen and I couldn't control it, and he never kissed or cuddled me again, and when mum did, she wasn't like a mum with a child any more, she was more just sort of – clinging onto me as if I was a teddy bear. And I wished they would kiss and cuddle me the way they used to, even though I'd never wanted them to before. And I taught myself not to be afraid of strange spells, by letting mum teach me how to brew potions when dad was out. So I suppose I was a different person.'

'I think you have answered some of your own questions,' said Mr Nutt. 'Even if you were born different from most children in some ways – having abilities that other children lacked, and lacking some of the skills that came easily to most of them – the capacities to love and be loved, to be a lovable person, to trust, and to be happy, were clearly there. And those memories, as much as the memories of all the pain and fear and confusion that followed, are part of what shaped you. And you were able to change and develop and overcome some of your earlier fears, so growing up cannot be simply a process of loss and shutting down of possibilities.'

'But I don't know how to be like that now! So I'd have to go back to being two years old – really two years old, not a toddler with an adult's memories and personality – to be able to be happy now.' He thought, again, of the memories they had watched of Konstantine as a younger man. 'I've seen you make right choices, under the most absolute stress.' 'But I can't remember them. Can't remember how I did it.' And also another memory – this time not a memory of Konstantine himself, but, again, of Aral talking about him.

'I saw a memory where someone said about Konstantine – that he couldn't ever be normal, but that at least finding him a job that was similar enough to his old job to give him some continuity enabled him to function. Is that all people like us can hope for? That we can be capable of doing a job, being useful for something, and that we can be contented if we don't bother wishing we could be whole?'

'"Normal" is not the same as "whole",' said Mr Nutt. 'And "normal" is a word that can be used in many different ways. If it means "typical of most people around you," then yes, many of us can never be normal. Just as having schizophrenia is not "normal", so being an orc is not "normal", and I understand that in your world, being a wizard is not "normal"?'

'Is it in yours?'

'It's – different. The Disc is a world much further Ayewards – more conducive to magic – than your world, so most people are born with some potential for learning magic, because there is so much background magical radiation. Also, Discworld wizards usually do not get married or have children, therefore Discworld witches, if they marry at all, get married to people who are not wizards and have children with them, so that the genetic predisposition for magic is more widely distributed. Of course, not everyone actually becomes a witch or wizard, just as not everyone in your world becomes a nuclear physicist, but innate magical ability is not the most important variable; witches select apprentices mostly for their strength of character, good sense, and willingness to work hard. And many of what would be considered "mundane" jobs in other worlds have a magical element…'

'Like Gytha Ogg's husband and son being blacksmiths who shoe Death's horse, and have an ancestral connection to the magic of iron, and can even shoe unicorns or ants or any creature?' asked Severus.

'Exactly. Discworld people born with no magical ability are as rare as non-magical swords. But I believe you know a wizard who is one of the few?'

'Rincewind?' It had been a long time since he had come to visit, and Severus hoped he would drop by again soon. He hadn't thought about the likelihood of Rincewind and Mr Nutt knowing the other, but, of course, Rincewind, when he wasn't trapped in the Dungeon Dimensions or being driven across the Disc by some terrifying adventure he had never wanted to get mixed up in, or staying safely out of harm's way on a remote island, lived at the same university where Mr Nutt used to work.

'The same,' said Mr Nutt. 'But I understand that in your world, "wizard" is not a job that a child can study towards or a man can retire from if he wants to get married, but simply the fact of what you are born as, just as I am an orc. Is that true?'

'Yes. Occasionally magical children suppress their magic, but it kills them if they do. I've heard of two who survived until they were in their teens, but they were unusual, and were very troubled and sometimes dangerous people.'

'So you are atypical in being a wizard in a Naywards world, and by that definition, you are "not normal". But would you want to be ordinary?'

'No.'

'And by the same token, perhaps you are atypical in other ways, without having anything actually wrong with you. Perhaps the same hypersensitivity which made you fearful of loud noises as a child gave you the attention to detail which enabled you to become excellent at brewing potions, for example.'

'If you're saying I'm autistic, forget it,' said Severus. 'An autistic person wouldn't have lasted five minutes as a spy.'

'Then probably you are not. Nevertheless, labels are an attempt to make the world seem simpler, but they are misleading if people assume that all people described by a label have the same characteristics. For example, not all autistic people lack a sense of humour. Not all autistic people hate physical contact. Not all autistic people read only non-fiction and can't see the point in fantasy.

'Still, let us say that you probably aren't autistic. At any rate, you said yourself that you were always different – more than in simply being a wizard – whether or not you have a precise label for the kind of "different" that you are.'

Maybe everyone who is different is a different kind of different. Severus wasn't going to say that aloud, as it sounded far too glib. But he remembered a time when Konstantine had been worrying about the way that he not only wasn't like a normal person, but wasn't even like a normal sociopath, compared to some of his friends on the Rock like Willikins, Commander Vimes' butler, or Amos Burton, the mechanic on the Rocinante. Willikins and Burton weren't ashamed of being illegitimate and not knowing who their fathers were, and never seemed to feel guilty or anxious about anything. Cheiron had said then that Amos, at least, wasn't nearly as different from Konstantine in that respect as Konstantine assumed, but that if there was anyone who genuinely was always calm, it didn't necessarily mean that they were any saner than Konstantine, but that they were just a different sort of different, had been born with brains wired in a different way which meant they didn't experience emotions, and needed to learn in a different way.

Author's note: Whitehound has said that 'Snape cannot be an Aspie, because in order to be an effective quadruple agent he has to have an ability to both read and deceive others on the fly which no Aspie would have,' but that, because the person he was based on was autistic and J. K. Rowling (who was a child when she knew him) did not understand this, she misinterpreted the original person's innocent tactlessness as malicious, and so wrote cruelty into Snape's character. So, even if Severus doesn't think of himself as autistic, there is no reason he couldn't have inherited other autistic-like traits. But in any case, as Nutt says here, all autistic people are different.

Whether Severus is autistic or not, it clearly isn't the sole reason for why he is the way he is, and we know that he had a traumatic childhood even before going to Hogwarts. But, as he isn't a sociopath, I want to think that, like Harry and unlike Voldemort, he had some good experiences in infancy before everything when wrong.

The terminology of Ayewards and Naywards worlds comes from the Magids series by Diana Wynne Jones.