Though what he really said is open to doubt, the nuclear physicist Enrico Fermi gave his name to a short and possibly final argument against the existence of intelligent life on other planets. There are 200 billion stars in our galaxy alone. Twenty billion of these are like our own sun. Let us assume that one in five of these has planets – and we find new exoplanets every year – and let us assume that one in a hundred of these one in five has one planet with liquid water: that gives us forty million earth-like planets. I will not carry on with the assumptions, but it seems reasonable that there should be around a hundred thousand other advanced civilisations in our galaxy alone.

This being so, the “Fermi Paradox” asks, where are they? So many other civilisations – so many of them presumably older and more advanced than our own – and they have not visited us. Nor, after generations of scanning with radio telescopes, have we detected any unambiguous signals from them. Either intelligent life on other planets does not exist, or it is so rare and so far apart in time or distance or both, that we shall never find it.

Writing in 2008, Nick Bostrom of Oxford University takes the argument to conclusions that are either depressing or exhilarating. He proposes a set of Great Filters, each of which limits the emergence of intelligent and technologically-advanced life. The most obvious filters are in the past. We shall soon be able to estimate how many planets in our galaxy have liquid water. We still have do not know how life begins. Obviously, it began here. But we have never been able to create a self-replicating organic process in our laboratories. It may be very unusual. It may also be very unusual, once begun, for this process to evolve beyond the very simple. Then it may be very unusual for larger and more complex living structures to evolve, and hardest of all for anything to emerge with the right combination of mind and appendages to enable the birth of a technological civilisation.

Or the Great Filter may be in the future. It may be that civilisations like our own are reasonably common – but that they invariably blow themselves up shortly after finding how to split the atom.

Bostrop’s conclusion is to hope that, when we get there, we shall find that Mars is, and always has been, a sterile rock. Independent life of any kind on a neighbouring planet would suggest a universe teeming with life, and some probability of civilisations like our own. This being so, the lack of contact would put his Great Filter in the future, and would suggest that we are, on the balance of probabilities, heading for self-extinction. No life at all on Mars, now or in the past, would let him keep hoping that the Great Filter is in the past, and that we may have a splendid progress before us.

The main counter-argument to the Fermi Paradox is that aliens have made contact with us. Since at least the 1940s, there have been thousands of reported sightings of unidentified flying objects and their crews. The problem with this counter-argument is that the claims of UFO sightings all appear to involve some kind of deception. Most lights in the sky turn out to have a human or astronomical origin. Most claims of physical contact are made by frauds or persons of unsound mind. In the past few days, I have watched dozens of YouTube videos that claim to show various kinds of alien encounter. Every one of them strikes me as fraudulent.

The wider claim of a conspiracy between governments and aliens can be dismissed at once. The most obvious deal between these parties is that our rulers give resources to the aliens, and they give our rulers a more advanced technology. But I can see no discontinuous leap in any technology. Everything we have now would have seemed marvellous to a man in 1948. But all of it has plainly grown out of what we already had in 1948. There are no warplanes with anti-gravity paint on them, no telepathy machines, no teleportation. Our rulers continue to get old and decrepit, and to die. Many people claim to have seen Elvis Presley since 1977. No one claims to have seen a rejuvenated and renamed Ronald Reagan or Margaret Thatcher, or any of the Rothschilds or Rockefellers. Bill Gates and Jeff Bezos and Elon Musk all look authentically past the bloom of youth.

Nevertheless, I do not believe that every reported sighting is untrue. I cannot say which ones are true. But I do find it likely that some are true. The problem with claims reported in the media is that they involve a mass of intellectual “white noise.” We live in a civilisation where primitive space travel is an established fact. We can easily imagine more sophisticated forms of travel between stars. We are willing to consider the possibility of life beyond the stars. Many of us want to believe in life beyond the stars. Therefore thousands of reported sightings that involve some kind of deception. How to decide if any may be true?

The answer, I suggest, is to look into the past. Despite a scientific consensus growing since before 1600, the general idea, until then, was that our planet was the largest object in the universe, and was at the centre of the universe. Angels might visit from the skies, but hardly anyone imagined there was any place above the skies from which natural beings might arrive. So, are there any records of UFO sightings from before our own Age of Science?

There are many. I will give four of these. I could give more, but have decided to give just four of those I find interesting and that I can be sure have not been forged in modern times. When I was a boy, I read and believed Erich von Daniken. I then discovered that most of his alleged evidence from ancient times was based on doctored or just fabricated documentation. I therefore take four records of sightings that are in Latin or Greek or French – that is, in languages I can read for myself – and that can be found in published texts of undoubted authenticity. For your benefit, I give both English translations and images of the pages where the originals can be found.

My first account is by Agobard, a bishop of the ninth century. Note that he refuses to believe the story he has heard. Note also, however, the claim of flying vessels, engaging in some kind of trade.

Plerosque autem vidimus et audivimus tanta dementia obrutos, tanta stultitia alienatos, ut credant et dicant quamdam esse regionem, quae dicatur Magonia, ex qua naves veniant in nubibus, in quibus fruges, quae grandinibus decidunt, et tempestatibus pereunt, vehantur in eandem regionem, ipsis videlicet nautis aereis dandibus pretia Tempestariis, et accipientibus frumenta vel ceteras fruges.
Ex his item tam profunda stultitia excaecatis, ut haec posse fieri credant, vidimus plures in quodam conventu hominum exhibere vinctos quatuor homines, tres viros, et unam feminam, quasi qui de ipsis navibus ceciderint: quos scilicet per aliquot dies in vinculis detentos, tandem collecto conventu hominum exhibuerunt, ut dixi, in nostra praesentia, tanquam lapidandos. Sed tamen vincente veritate, post multam rationicationem, ipsi qui eos exhibuerant, secundum propheticum illud confusi sunt, sicut confunditur fur quando deprehenditur.

We have seen and heard many overcome by such madness, separated by such stupidity, that they believe and say there is a certain place called Magonia, from which [place] ships come through the skies, in which [ships] are carried back to that place [Magonia] the crops which were ruined by hailstones and lost in storms – these sky-sailors making payment to the Tempestarii [Masters of the Storms?], and having in exchange wheat and other crops. From these [people] so blinded by deep stupidity, that they are able to believe such things can happen, we have seen many at an assembly, showing off four captives – three men and one woman – as having fallen from these ships. These they showed in chains for several days in this assembly, as I have said, in my presence, saying that they should be stoned to death. But, truth overcoming them after much debate, the people who had shown the prisoners were confounded, in like manner to the words of the prophecy, that the thief is defeated when captured. (Bishop Agobard of Lyons, d. c840, “Liber Contra Insulam Vulgi Opinionem de Grandine et Tonitruis,” c.II – given on p.148 in Patrologiae Latinae, Vol. 104, Migne Edition, Paris, 1864)

My second account is by Gervase of Tilbury, writing in the early thirteenth century:

Accedit ad probandam maris superioris supereminentiam temporibus nostris novum divulgatum, tamen mirabile. Cum enim die festo in majori Britannia populus post audita Missarum solemnia ecclesiam plebanam passim egrederetur tempore quidem plurimum nubilo ac propter nebularum densitatem subobscuro, apparuit ancora navis, lapideo tumulo infra septa circuitu infixa, fune in aëre protenso ac pendulo. Obstupuit populus, dumque super haec varia inter se conferrent, tandem viderunt funem moveri quasi laborarent ad evulsionem ancorae. Dum ergo plurimum proficiata non cederet, audita est vox in aëre spisso velut clamor nautaram ad revocandam projectam anchoram contendientium. Nec mora: frustrata laboris spe, nautae unum de suis dimittunt, qui secundum peritiam nostrorum funi anchorali impensus vicaria manuum permutatione descendit. Cumque jam anchoram evulisset, a circumstantibus capitur et inter manus contrectantium, quasi in mari naufragium faceret, crassi aëris nostri humectatione praefocatus exhalavit. Sed et superiores nautae, arbitrati socium naufragatum, post unius horae spatium funem anchoralem praeciderunt, relicta ancora navigantes. In hujus itaque rei memoriam de anchora illa ferramenta ostii basilicae illius prudenti consilio fabricata sunt, quae publico patent conspectui.

In our own day appears new corroboration of the greatness of the sea which is above the sky. This is well-known, but wondrous even so. On a feast day in Great Britain, when the people had finished attending a service and were leaving the church, and it was dull and dark outside, because the sky was covered in dense cloud, there appeared the anchor of a ship above the tombstones. Its hook stuck under a fence, and the stretched cable went high into the sky. Everyone was amazed by this vision, and spoke much about it. At last, they saw the cable begin to move as if someone were trying to move the anchor. When the anchor remained stuck, a voice was heard in the heavy air as of sailors when they seek to recover a stuck anchor. Without delay, the work going nowhere, the crew chose one sailor, who came down the cable. He came down hand-over-hand just as our own sailors do. As soon as he let go, he was seized by those who were standing close by. He died in their arms, suffocated by the damp of our heavy air as if drowned in the sea. The sailors who remained above decided that their companion had drowned. After one hour they cut the cable and sailed away leaving the anchor behind. Afterwards, following prudent advice, it was decided to make iron fittings for the church doors out of this anchor in commemoration of the event. They can still be seen there. (Gervase of Tilbury, c1200, Otia Imperalia, Hanover, 1856, pp.2-3)

My third account is by Jacques Fodéré, a French churchman of the early seventeenth century:

In the year 1603, being in Besancon for the duties of my charge as Visitor to Sainte Claire monastery, it happened that on a Thursday, the 23th day of January, between 7 and 8pm, we were told that all the people were assembling in the streets, terrified. I went out, and like the others I saw a great light in the air over the cathedral, covering the whole of Mount Saint Etienne with a round-shaped, heavy cloud, reddish in colour, while all the air was clear and the sky so devoid of fog that the stars were seen shining brilliantly. This light remained quasi-motionless over Mount Saint Etienne, and from there we saw it coming so low that it nearly touched the houses and lit up the nearby streets, but with a motion so slow that it was hardly noticeable, and it halted for at least a quarter of an hour over Saint Vincent Abbey, where some pieces of relics of two glorious Saints are kept. Then, escaping over the Grande place of Chammar to the river Doux, it went away through the Grande rue that goes to the bridge, and straight to the cathedral where it vanished, but as we said before, with such a slow motion that its travel lasted until 9:30 at night, which is to say at least two hours. (Jacques Fodéré, Narration Historique et Topographique des Convens de l’Ordre de St-Frangois… Lyon, 1619, pdf pp.1059-60)

My last account is the earliest of all. I found it in one of the epitomes of the Historiae Romanae of Cassius Dio, a Greek historian of the third century, who is writing about an event from 193 AD, near the end of the brief reign of the Emperor Didius Julianus. I take the text from the on-line Perseus edition (bk.74,cc.14-15)

καὶ τούτους ἄρα οἱ ἀστέρες οἱ τρεῖς οἱ ἐξαίφνης φανέντες καὶ τὸν ἥλιον περισχόντες, ὅτε τὰ ἐσιτήρια πρὸ τοῦ βουλευτηρίου ἔθυεν ὁ Ἰουλιανὸς παρόντων ἡμῶν, ὑπῃνίττοντο. οὕτω γὰρ ἐκφανέστατοι ἦσαν ὥστε καὶ τοὺς στρατιώτας συνεχῶς τε αὐτοὺς ὁρᾶν καὶ ἀλλήλοις ἀντεπιδεικνύειν, καὶ προσέτι καὶ διαθροεῖν ὅτι δεινὸν αὐτῷ συμβήσεται. ἡμεῖς γὰρ εἰ καὶ τὰ μάλιστα καὶ ηὐχόμεθα ταῦθ᾽ οὕτω γενέσθαι καὶ ἠλπίζομεν, ἀλλ᾽ ὑπό γε τοῦ παρόντος δέους οὐδ᾽ ἀναβλέπειν ἐς αὐτούς, εἰ μὴ παρορῶντές πως, ἐτολμῶμεν. τοιοῦτο μὲν τοῦτο οἶδα γενόμενον

And three stars that suddenly came in sight surrounding the sun, when Julianus – ourselves [the assembled Senators] being present – was offering sacrifice in front of the Senate House, suggested [the success of] these men [three rival candidates for the Purple]. These stars were so clear that the soldiers kept looking at them and pointing them to each other, saying that something terrible was about to happen to him [the Emperor]. We were hoping it would be so, but we dared not on account of present fear to look at them, except by brief glances. This incident I know for myself.

These accounts are widely-spaced in time. They seem to owe nothing to each other. They prove no theological point. One of them is sceptical. Two speak of ships in the sky. One of them speaks of a crew member unable to breathe our air. The third records no sighting of a solid object, but does record phenomena that have no obviously natural explanation. This third, indeed, is by a man of reputation, who records what he claims to have seen with his own eyes, and who published just seventeen years after the alleged sighting, when many of the other witnesses he mentions were still alive to dispute or even deny his record.

The last account, from Cassius Dio, is less authentic. Cassius claims to have been present at the sighting. However, he died in 235, over forty years after the alleged sighting, and the account is taken from what is probably one of the last completed sections of a vast work that describes the whole of Roman history. Also, this particular book survives only as a set of extracts from much later writers. It is therefore not a contemporary account, and what we have may not be in his own exact words. Even so, we have the sighting of three bright objects appearing suddenly about the risen sun. There is no convincing modern explanation of the appearance. Nothing is said explicitly about their origin, and the account of how he and his fellow Senators dared not look too long at them, but how the soldiers found them entertaining, has a ring of truth.

The alleged facts in all of these accounts are open to question, but not the authenticity of the accounts themselves. It is conceivable that the books are clever forgeries of the past few years. Bearing in mind, however, the shortness of the accounts within their surrounding material, and how little they say compared with what a forger might wish to say, I think this hypothesis needs no further discussion. I will add that, if less authentic than the others, the story in Cassius Dio is taken from a standard text, and any interpolation would raise an immediate public scandal. It is a rebuttable presumption, therefore, that these are garbled accounts of UFO sightings. If I had the relevant language skills, I have no doubt I could find similar accounts in Arabic and Chinese. I will not look for these, because I am unable to examine them for myself. But the Latin and Greek and French are enough for me.

Why might aliens be interested in us? That is a hard question, and I will leave it aside for the moment. Why they seem to have taken less care in the past to hide themselves than nowadays is easier to answer. When I was a very young boy, the women in my family would get dressed and undressed in front of me, confident I would never remember anything when I grew older. As I did grow older, they behaved with greater modesty. In the same way, it makes sense that an alien mission would show itself to intelligent but untechnological beings, making greater efforts at concealment only when these beings began a rapid course of technological progress. What these aliens might be doing here is unknown. But the evidence that they are here begins to mount long before the 1940s.

Now, an objection to these texts is that they are taken from longer works filled with other marvellous accounts that I would dismiss out of hand. Am I not giving my belief in an arbitrary way? I do not think I am. If a monk is said to have converted the heathen by cooking meat without fire, or by raising a man from the dead, this can be dismissed. Our knowledge of how the world works has no room for miracles. But there is nothing miraculous about a UFO sighting. Even if no account is actually true, UFO sightings have a natural explanation. As said, if a man says now that he saw a flying saucer, our default response should be sceptical. When a writer from some more remote age reports a sighting, what he says should be taken into consideration.

Another objection to my general point is that the texts may record folk memories of a long-vanished human civilisation on this planet. The flying ships may be distant echoes from Atlantis. There is an easy reply to this objection. Since about 1500, we have made obvious and irreversible changes to the planet. We have introduced tobacco and tomatoes to Europe, and horses to South America and rabbits to Australia. We have extracted all the mineral resources that can be easily got at. If the human race vanished tomorrow, and all our cities fell to dust or were overspread by jungle, it would still be obvious to any alien visitor that the Earth had once supported an advanced civilisation. The fact that we took control, after 1500, over a virgin planet indicates that ours is the first advanced civilisation on this planet.

Yet a further objection is that the accounts given above may be true, but that my interpretation is limited. I have a taste for science fiction, and this may set me to explaining these accounts in terms of visitation by aliens similar to ourselves but more advanced. Jacques Vallée began his own research by accepting the same assumptions as mine. He then rejected these in favour of suggesting that the visitors are not natural beings from another planet, but multi-dimensional beings outside time as we understand it. He summarises his evidence thus:

1. unexplained close encounters are far more numerous than required for any physical survey of the earth;
2. the humanoid body structure of the alleged “aliens” is not likely to have originated on another planet and is not biologically adapted to space travel;
3. the reported behaviour in thousands of abduction reports contradicts the hypothesis of genetic or scientific experimentation on humans by an advanced race;
4. the extension of the phenomenon throughout recorded human history demonstrates that UFOs are not a contemporary phenomenon; and
5. the apparent ability of UFOs to manipulate space and time suggests radically different and richer alternatives.

I admit that I dislike his explanation because I prefer my own. Again, though, I see no good reason for an appeal to what borders on the supernatural when a natural explanation is available. I accept his fourth point, but reject all the others. As said, the great number of modern reports can be dismissed as untrue. This also takes most reports of eccentric or marvellous behaviour by the visitors. As for reports of humanoid body structures, we have as yet absolutely no comparative biology. It seems likely that most life is carbon-based. It is possible that technological progress is most likely among beings that share our general size and shape. I will not say that beings without arms and legs, or that look like centipedes, are incapable of developing a high technology. But we are developing a high technology, and our particular size and shape seem to have made this an almost expected achievement. Why should alien visitors not look something like us? That might even explain their interest in us.

So far, I have refused to speculate on why we have been and continue to be visited. This is because I see no obvious explanation. But I think we can reject any malevolent purpose. If they wanted it for themselves, the aliens would had done better to take our planet before our own industrial progress had raised the cost of extracting resources. Also, there are no inorganic materials on this planet that are not abundantly available everywhere else in the universe. Perhaps they are waiting for us to cross some technological threshold, at which point they will destroy us as a potential threat. Again, I see no reason for this. If the threshold is nuclear fission or space travel, we passed that point some while ago, and our progress has been upward ever since. If the threshold is interstellar travel, that seems to be a very long time to go – and it might then be too high a threshold, as that sort of technology could make us a formidable enemy.

We can also reject any overt benevolence. It is possible that some external push moved us from apes with large brains to human beings. Though related, we are plainly different from all other creatures on our planet. We have uniquely powerful minds. Our voice boxes are unique – so too our manual dexterity. But these could just as easily be accidents of evolution. Returning to the von Daniken school of explanation, it is clear that we were intelligent enough to build the pyramids by ourselves and to develop all the other interesting things the archaeologists keep finding. If I were a benevolent alien, I might show my subjects how to harness farm animals without strangling them, or something about germ theory. Putting them to the vast opportunity cost of heaping up artificial mountains would be at least low on my agenda. Perhaps they are waiting passively for some threshold I have overlooked, at which point they will reveal themselves and invite us to join some interstellar federation of peace-loving vegans. Perhaps they are, though I doubt it.

The most likely explanation, if one is required, is that they watching us out of scientific interest. Though vain to say, I am sure we make good theatre. They seem to have been watching us for a long time. Every so often, they seem to have shown themselves by other than accident. Perhaps they have monitored our reactions. Perhaps they will eventually make contact. Or perhaps they will grow bored, or finish their research, and move somewhere else for their studies, taking with them all evidence of their base on Mars or the dark side of the Moon. I really cannot say.

But I will conclude. There is good reason for rejecting the Fermi Paradox. There are reasonable grounds for accepting a continuous alien presence. When some American of low intelligence or a history of prescription or recreational drugs says he was abducted into a flying saucer, there to be examined or sexually molested, I am sceptical to the point of incredulity. When I see probably untouched accounts from the past of flying ships or radiant lights in the sky, I will pay attention. Those accounts raise an arguable case. Their number and consistency places the burden of proof on the doubters. The universe may well be teeming with life, and we may eventually find ourselves actively aware of it.