'Zoe!' she heard the Doctor say under his breath. 'I need to know about camels.'

'Caramels: chewy candy made of sugar, b.u.t.ter and milk.'

'Not caramels, camels.'

'Ca.n.a.ls: manmade waterways-'

'Not ca.n.a.ls, I said tell me about camels!'

'Doctor,' Peri said sweetly, 'who are you talking to?'

He started guiltily, hurriedly hiding something behind his back, and a look Peri had never seen before crossed his face. After a second she realised that the Doctor was actually looking sheepish. Well, she guessed there had to be a first time for everything.

'It's Zoe,' he said.

'What, as in Jamie and Zoe?' (The Doctor had given her a crash course on all his former friends. It had saved her life once on Karfel.) 'No Zoe as in personal a.s.sistant and knowledge interface. Although I did name it after Zoe as in Jamie and Zoe. Zoe knew about everything, you see. Well, except candles.' He drew his hands from behind his back and Peri saw he was holding a flat rectangular device. 'This is called a tablet. No, not like Tylenol, as I'm sure you were about to say.' (She hadn't been. She might not be that up on future stuff but she was pretty sure you wouldn't try to swallow an electrical device for a headache.) 'And it's some amazing Time Lord gadget?'

'Not exactly. I bought it on the net in 2015.'

'What, like a fishing net?'

The Doctor was just about to answer when a cry reached them.

'Quick, upstairs!' yelled Peri. She hitched up her skirts and ran up the stairs in a way that would have shocked London to its core if anyone but a very rich person had done it. The Doctor followed. They burst into Jane's room...

... and saw Miss Hyde the governess standing over the unconscious girl, capturing in a vial the purple ribbons of light emanating from her mouth.

'So, we meet at last, Potentialiser!' declared the Doctor.

'Pot-what-now?' said the Potentialiser.

'It's our name for you,' Peri explained.

'Mm, I quite like it,' said the Potentialiser. 'Do you spell it with an "s" or a "z"?'

The Doctor ignored that. 'I am going to stop you, Potentialiser,' he said. 'No more poor souls will fall victim to your evil machinations!'

'What's a machination?' asked the Potentialiser.

'Machination: a crafty scheme or plot for sinister ends,' said Zoe.

'I don't have any sinister ends!' said the Potentialiser.

'You murder people!' said the Doctor.

'I do not!' said the Potentialiser.

'Shall we all sit down and talk about this like sensible people,' said Peri.

Luckily, everyone agreed.

'So draining people's potential doesn't actually kill them, then?' said Peri, pouring the Potentialiser a cup of tea.

'Of course not!' she replied indignantly. 'What do you think I am?'

'And you're not doing this for money? You're not selling skills to the highest bidder?'

'Of course not!' she repeated, still indignantly. 'After extracting the unused the unusable potential energy, I spend a lot of time finding a suitable vessel for it. You see,' she said, helping herself to a Bath bun ('a sweet roll made with yeast and topped with sugar, sometimes containing dried fruit,' pointed out Zoe), 'I just couldn't stand the waste. All that potential for the benefit of mankind, left to wither in infertile soil.'

'But you kill people!' Peri insisted. 'The little chimney sweep the caveman...'

The Potentialiser raised an eyebrow. 'I do not! Do you know the morbidity rates for chimney sweeps and cavemen?' ('Searching: morbidity rates for chim-' began Zoe. 'Shut up!' shouted everyone else.) 'Look, I don't kill people. But sometimes, I'm afraid, they die. That's what history's like. Death taking place after I've visited is just a coincidence.'

'I don't believe you,' said Peri. 'Look at Jane. Since we arrived she's got really ill. Vomiting. Sore throat. Aches and pains...'

'Symptom checker: consistent with influenza, food poising, a.r.s.enical poisoning...' came the tinny voice from the Doctor's tablet.

a.r.s.enic! There was something at the back of Peri's mind...

'Zoe, can you tell me about uses of a.r.s.enic in the Regency period?'

'Uses of a.r.s.enic: rat poison, wallpaper dye, facial creams...'

Face cream! Jane had been using face cream to make herself look good for Peri's wedding. She was only dying because Peri had infiltrated her house in order to save her life. That was a bit awkward.

They were back at the TARDIS. Yvette the aristocratic lady's maid had been given a lecture on how not to kill people with beauty aids and Jane was quite recovered. Peri had broken the news to Lord Roderick that she couldn't marry him because their respective countries were about to go to war (Zoe helped with that t.i.tbit), and he was quite sad about it until she told him she'd also lost all her money, and then he went off happily to do something loud with dogs or possibly horses. The Potentialiser had tried to explain to Peri what an anpholier was and how she'd make sure Jane's gift made life infinitely better for a lot of future people, but, even with the aid of Zoe, Peri failed to understand the explanation. And as for Zoe...

'Look, I don't think the TARDIS is big enough for two know-it-alls,' she told the Doctor.

'Oh, I'd hardly call you a-'

'I meant you, and you know it. Look, it's Zoe or me. And the answer had better be me.'

The Doctor had that sheepish expression on his face again. Twice in one day! Peri would have noted it in her diary, if it wasn't that time travel made keeping a diary a sort of chronological Russian Roulette. 'The trouble is, it had become expected of me that I know everything,' he said. 'And it turns out that actually, I only know nearly everything. Zoe helped me have the facts at my fingertips, so to speak.'

'Yeah, but half the time she got it wrong. Mozarella? Camels?'

'She just misheard me, that's all. Or I misheard her.'

'Look, it's the computer or me.'

'What do you think, Zoe? Her or you?'

'I'd keep her, Doctor,' said Zoe. 'She's much prettier than a computer.'

'All right,' said the Doctor.

'Earth. England. Sherwood Forest. 1190 AD ish. But you'll only be disappointed. No damsels in distress, no pretty castles, no such thing as Robin Hood.'

The Twelfth Doctor, Robot of Sherwood (2014)

Is history a science? That is something that's long been argued about. The word history comes from the ancient Greek 'historia', meaning inquiry or knowledge acquired by investigation. Just like science, history depends on building up theories from evidence. But there are different kinds of evidence. It's an important principle of our legal system that everyone is innocent until their guilt is proven beyond reasonable doubt by the weight of carefully scrutinised evidence. Forensic science may be used to gather that evidence, but we don't usually think of law more generally as a type of science.

Historical evidence can take different forms. In fact, the past is all around us: we remember things from the past, while people older than us can remember bits of the past from before we were even born. Our language, culture and knowledge, even the landscape where we live, have all been shaped by people now long dead. But we can use what survives from the past in our attempts to understand history.

For example, think of the people in charge of the costumes and sets for making the Doctor Who story Robot of Sherwood, who needed to know about life in the year 1190. The evidence for that might include physical remains that have survived into the present day. They could study the ruin of a castle from the time and deduce from that how its inhabitants lived. Archaeology in the castle grounds might have found artefacts such as the remains of weapons, tools or jewellery that give additional glimpses into what daily life might have been like exactly the sort of thing that could be reproduced as props in the Doctor Who story. Scientific tests on even fragments of skeletal remains found in the castle might reveal medical information such as the person's age, s.e.x, level of fitness, diet and even how they died. Again, these scientifically gathered details can help build up a vivid picture.

But, broadly speaking, evidence in history usually means written accounts by people who lived at the time. Written evidence is so central to our idea of what history is that the word 'prehistoric' is used for that period of time before the invention of writing. And written texts present a number of problems as evidence.

For example, to know about 1190, we could refer to the five-volume History of English Affairs by William of Newburgh, which details life in England between 1066 and 1198. It is thought that William wrote the book in the 1190s, so may have been a first-hand witness to at least some of the later events he describes. Historians particularly value his book because of its detailed account of the crisis that followed the death in 1135 of King Henry I without having a legitimate son to take the throne after him. William gives a good sense of the political intrigues and battles as Henry's daughter Matilda fought her cousin Stephen for the crown.

But how reliable is the evidence presented in this history? We don't know how much William of Newburgh himself witnessed the things he described, though it's generally thought he copied much of his information from a number of other accounts that have since been lost. That makes it difficult to know how accurately he described what happened or whether it happened at all. We know very little about William of Newburgh himself we're not even sure that that was his name. A copy of William's History of English Affairs now in the British Library contains corrections in William's own hand, so we at least know that the history we can read today is as he wrote it at the time (and hasn't been amended by later writers, as has happened with other old texts). But we don't know if he or his sources of information were biased in favour of one side in the crisis or other which might mean exaggerating or making up stories. Some of the other things in the history are certainly so extraordinary we might question whether they're true: the strange green children from 'St Martin's Land' who appeared in the Suffolk village of Woolpit; the bishop, Wimund, who became a pirate; or what the book describes in Latin as 'Sanguisuga' blood-sucking creatures that might be some kind of vampire.

We can only judge the quality of the evidence presented in William's history by comparing it with other evidence from the time without knowing if they are any more or less reliable than his. In fact, one reason he's often thought to be a good historical source is that he simply tells us that he is, while pointing out errors and fabrications in the works of other writers. But he would say that, wouldn't he?

As a result, some people argue that history isn't really a science because it's all about forming an opinion on the relative value of different sources, which is too dependent on personal interpretation to be called truly scientific (although even evidence that is gathered scientifically needs interpretation). Just as in the story that preceded this chapter, history is about more than a statement of simple facts the years a person lived, a one-line summary of their life. It's about understanding that life in context, and about how the past affects us today.

'Doctor, will he [Richard the Lionheart] really see Jerusalem?'

'Only from afar. He won't be able to capture it. Even now his armies are marching on a campaign that he can never win.'

'That's terrible. Can't we tell him?'

'I'm afraid not, my dear. No, history must take its course.'

Vicki and the First Doctor, The Crusade (1965)

We saw in Chapter 6 that, despite what the First Doctor tells Vicki, history can be changed in Doctor Who. But Doctor Who stories can also show us some of the different ways that we interpret history and how our sense of history changes. We're told in Robot of Sherwood that 'King Richard is away on crusade'. In fact, the series had already shown us King Richard the First Doctor meets him in Palestine in The Crusade. The script for that story doesn't give us the precise date this meeting takes place, but tells us that it is some time between the Battle of Arsuf (7 September 1191) and Richard seeing the city of Jerusalem (in June 1192).

Though The Crusade and Robot of Sherwood are set in roughly the same year, they present the past very differently. The Crusade is in black and white, the pace is much slower and we can see that it's been recorded in a television studio the forest in which the TARDIS lands is clearly indoors, on an artificial set. The Crusade feels like a televised play, where Robot of Sherwood is more like a movie.

These things are obviously because The Crusade was made fifty years ago, when technology was less advanced. But something that isn't down to technology is the way King Richard is played. For example, when he first meets the Doctor, Richard refers to the situation back in England but note the style of language he uses:

'And now I learn my brother John thirsts after power, drinking great draughts of it though it's not his to take. He's planning to usurp my crown, and trade with my enemy, Philip of France. Trade! A tragedy of fortunes and I am too much beset by them. A curse on this! A thousand curses!'

Richard the Lionheart, The Crusade

It's an old-fashioned way of speaking, very different from the way people speak in Robot of Sherwood. From what we've learnt in other episodes, we could perhaps argue that that's the way the TARDIS is translating Richard's words since, as with all English aristocrats of the time, the real Richard would actually have spoken a form of French. But perhaps David Whitaker, the writer of The Crusade, is doing something else: the way Richard speaks sounds almost Shakespearean.