A/N: I was pretty horrified by the Baskerville experiment, and I really think John should have been angrier. It took me a while to reconcile Sherlock's apparent heartlessness with his little shows of emotion. Some deduction and psychological research sorted that out. Usual disclaimers.

'YOU'RE A GENUINELY HEARTLESS BASTARD, YOU KNOW THAT?' John was angry, in case that wasn't clear.

'Of course I know that. I said I'm sorry.'

'You don't know what that means, do you?'

'You know I do, how many dictionaries do I own?'

'Don't you dare quote the Oxford English "sorry" at me! What do people mean when they say it? You don't know, do you?' The boy's noses were inches apart, but the ample tension was not exactly sexual.

'They mean that they're willing to sacrifice their own pride in order to ingratiate themselves to the other person.'

'Give up their pride? That's what it's about to you?'

'It is to everyone and you know it, don't get sentimental, it doesn't help. "Sorry" implies regret, regret implies misjudgement, it all comes down to admitting you're wrong.'

'Which you won't do. Ever.'

'Well I wasn't wrong.' Sherlock was infuriatingly calm and confident.

'Then you're not really sorry, and I'm still just as angry.'

'So you should be. It looks good on you.'

'How can you not be affected by this? Why don't you care about ANYONE?'

'I've been thinking about that, actually. 'I'll get back to you when I have an answer.'

'That'll be the day.'

'John, you haven't spoken to me for seventy-seven hours, I think it's about time I got back to you.'

'Go on' he replied wearily.

'I think it's genetic. Mycroft obviously has it. It's like we have some sort of protection. A barrier between observing stimulus, say, a death, or a pretty woman, and reacting chemically to it. Our minds are free from the adrenaline and oxytocin, respectively, that initiate the emotional reactions experienced by everyone else. We don't have the internal chemical addictions everyone creates for themselves. We really should be more thankful for it, it's quite a gift.'

'Bullshit.'

'Sorry?'

'I call bullshit. On your whole theory. '

'Prove it.'

'You respond emotionally, I've seen it.'

'I was faking it. When?'

'When you were staring at that fake painting, we a heard the kid's voice, you went into overdrive, you cared more about a kid dying than an adult, tell me that's not sentimental. No, don't, I haven't finished. Whenever someone threatens Mrs Hudson, you go all fatherly. Don't argue! When you brother was rude to her, you jumped in as soon as I did.'

'Before.'

'Exactly. Someone gaged her, you subdued him and could have just waited for the police, instead you threw him out a window... And what about me?'

'What about you?'

'Every criminal in the city seems to know how to get to you, they put a gun to MY head. It's not an original trick anymore, it's a standard routine, why do you think that is?'

'Because it works.'

'Exactly! Premise one, Sherlock has no emotional response, neatly disproven...

'Now, wait...'

'No! I put up with mountains of your crap, it's my turn! Not only do you have emotions, you have hormone addictions, worse than anyone I know. If I don't get laid for a month, I get a bit grumpy. If you don't have your adrenaline hit for one day, you shoot out the walls!'

'So what's your diagnosis, oh great doctor?' Sherlock asked sarcastically.

'This is about negative and positive...'

'Oh, I...'

'Shut up. The normal brain gives instigative and prohibitive commands. The former tell you you want to kiss pretty girls, the latter tells you not to kill people.'

'And?'

'You don't have the latter. Your brain never punishes you for doing anything. It just encourages you. You have no negative commands, your mind never says "don't say that, it's offensive" or "don't hurt him, it's unkind". You have no idea why you shouldn't use me in your experiments, because no one tells you.'

'So who says I have the former?'

'I do. Your mind says "shoot this, it'll be fun" and "smoke this, you'll enjoy it" and even "save him, you love him".'

'Love is a chemical weakness that affects...'

'You.'

A/N: Does this need to resolve in a physical way on order to be narratively satisfying? I wasn't sure.

Also, does this need more context (ie, who says what), or is the meaning clear?