Six Science Questions – Answers from the Sixth Century

Even in the Early Middle Ages people were asking scientific questions about their world. Here are six of these questions, and the answers that were provided by a Byzantine philosopher in the year 531.

The Answers to King Khosroes was written by Priscian of Lydia, who was a philosopher working in Athens in the sixth century. When the Byzantine emperor Justinian I closed his school in 529, as part of his efforts to end the teaching of pagan ideas, Priscian and several other thinkers took refuge with Khosroes I, the king of Persia. It was here that Priscian recorded his replies to Khoroes’ questions about a wide range of matters. Here are six excerpts from his work, where the philosopher expounds on the insights of ancient thinkers.


Why do we sleep?

The cause of sleep in animals is food coming in from outside, and an excess of the fluid and the hot inside arising from certain causes. For from such food present in the receptive areas, a vapour develops, which passes into the veins and is thence carried to the head. For it is necessary that what is pushed up is pushed up only so far, and thence returns and changes course. Moreover, what is hot in any animal tends to rise to the upper parts and there it befalls that matter which has become heavy in accumulating turns back of its own accord, is carried downwards and chills the heat which is around the heart. Sleep is induced when such chilling takes place. For the vapours from fluid, spreading to fill the upper parts around the brain, into which they gather – and which is the coldest thing of all in the body – weigh down the head and the eyelids  and make the individual sleep. 

Sleeping bishop -British Library MS Royal 10 E IV f. 241

How can you remember your dreams?

Dreams become either more disturbed or clearer appropriately according to the time or how the body is positioned or lies. They are confused and false around spring or autumn, as they are immediately after a meal. Morning dreams, as the disturbance ceases, are clear. Again, those who lie on their backs dream; those who sleep on their fronts are in a good position, that is to say they dream less.


Why don’t the seas get larger?

Why, when every day countless mighty rivers flow into it, the sea is no way becomes greater, is a question which it is not unreasonable for some people to have failed to answer, but which is not difficult to understand, if you look closely. For if the same quantity of water is spread out over a wide area, or gathered together, it does not dry out int he same time-scale. There is a difference: the former takes a whole lunar day to evaporate, while water poured into the sea disappears just as much as if you were to pour a cup of water over a big table: it disappears as soon as you notice it. This also happens with rivers. Since they flow continuously and accumulate, whatever reaches some vast and flat area is quickly and unobtrusively dried. For since many vapours have been drawn up from the sea by celestial bodies, especially the sun, the water that comes from the river is no more than what has evaporated. 

Tidal wheel showing the thirty-day lunar cycle in correspondence with the tides, with the earth at the centre of the diagram and four circles in the outer corners representing the four moons. British Library MS Harley 3017 f.135

What causes tides?

Priscian first notes that he is basing his answer on the philosopher Posidonius, who explained the both the sun and the moon have an effect on water:

The sun’s fire, he says, is pure and of great power; so however great the vapours it raises from land and sea it also subsequently destroys them by its fire. The moon’ fire, on the other hand, is not pure,  but weaker and feeble and therefore more productive for things on earth. However, it cannot consume what it produces, but only raise up, and makes waves in, bodies of water, disturbing them by its heat, but not diminishing them because of the weakness of its heat and its greater fluidity…

So also the water of the sea goes round the moon. As if raised and thus weakened by the moon, it rises to a flood and then when the moon declines towards its setting, it declines with it. It does this even when the moon goes beneath the earth on a daily basis.


Why do doctors prescribe different medicines?

Priscian explains that when someone becomes sick:

…the doctors who visit the patient agree in declaring that it comes from cold or heat, but in curing and in the chosen strength of medicaments they are at odds; and one believes that the giving of one medicament is harmful and counter-productive , but another, bringing what in his opinion is helpful, cures the patient? And if indeed medicaments are contrary to one another and do not shorten a weakness, why does a sick man with their help become healthy?

His answer:

And from this you should learn against disagreement between doctors, that there is indeed a multitude of them, but you should understand that there is just one real craftsman, and similarly just one discipline of medicine. No more will you dispute with the sick about differences between remedies or the application of suitable medicaments and their consequences, as these matters have been completely done away with, it seems to me. But the skilled doctor with diligent knowledge, by himself using the discipline and his experience, will know the differences between medicaments, just as also each of their powers. Whenever he knows atmospheres and localities and the variation of waters, and is helpful in weaknesses by knowing from nature the quality of the things that heat and make things cold and have a moderate blend by weighing the amount of each thing and what is the efficacy of simples when mixed with others and when given by themselves, then too at no time in no way will he stand in the way of individual needs either by fitting uniform cures to the continuously flowing and mobile matter of our bodies, or by applying to different types of patient cures mixed accordingly to one fixed measure and standard weight.

What is the climate of the Earth?

It must be known that those who say the earth is spherical suppose that there are five zones on the earth: they called the two extremes at each end uninhabitable as par excellence frozen and cold; for as they are under the poles themselves which hold the extremities of the sphere on each side, they are naturally far removed from the route on which the sun makes its journey: but the middle one of these five is burning hot, not habitable as being under a blazing burning, reaching from one tropic to the other; but two along are habitable, of which each is located parallel between one of the cold ones and the middle fiery one, as if, by receiving at their extremities the frozen and hot air, tempering the air in their own region; the northern one is inhabited by us, but the southern one by others opposite to us.


You can read more of Priscian: Answers to King Khroses of Persia, translated by Pamela Huby, Sten Ebbesen, David Langslow, Donald Russell, Carlos Steel and Malcolm Wilson, which was published by Bloomsbury Academic in 2016.

Click here to learn more about the book from the publisher’s website

Click here to buy this book from

Top Image: Map of the earth surrounded and divided by the ocean into two hemispheres with two uninhabitable zones at the extreme north and south. British Library Harley 2772 f.70v