Using dreams to diagnose diseases: why?

Hippocratic text: On Regimen IV

Some ancient medical writers recommended the use of dreams to diagnose diseases and to verify that our body was healthy. We see this especially in the Roman Imperial period: for instance, in On Diagnosis from Dreams by Galen (129 - 216 AD). One of the earliest texts in this tradition is from the period of Classical Greece (510 - 323 BC): the fourth book of On Regimen.

We don’t know exactly when this text was written, but it was most likely in the late 400s BC. We also don’t know who wrote it. It was attributed to Hippocrates in antiquity, for which reason we say that this book is ‘a Hippocratic text’. It belongs to a series of dozens of such texts, and the total body of these texts is called the Hippocratic corpus.

The author makes such pronouncements as the following:

“Such dreams as repeat in the night a man’s actions or thoughts in the day-time, representing them as occurring naturally, just as they were done or planned during the day in a normal act — these are good for a man. They signify health, because the soul abides by the purposes of the day and is overpowered neither by surfeit nor by depletion nor by any attack from without” (On Regimen IV §88).

In other words, if your dream matches what you did during the day, you’re healthy. But, the author continues, if it doesn’t, then something is wrong in your body (§88). If your dream features a violent struggle, that means there is a violent struggle happening in your body. In that case, the author recommends taking an emetic, eating lightly for a few days, and walking in the morning (§88).

Fruitless trees signify the corruption of one’s sperm. If the trees are shedding their leaves, then the problem is caused by too much cold. If the trees have plenty of leaves but still no fruit, then the problem is caused by heat (§90).

If you dream of rivers with overflowing waters, then you have too much blood. If you dream of rivers with low water levels, you have too little blood. Impure streams indicate disturbances in your bowels. In this case, the author thinks that you should run and walk a lot, as well as quicken your breathing in order to remove impurities (§90).

Most of On Regimen IV is like this: a manual of how to interpret dreams. In these sections, which comprise about 95% of the text, or maybe even more, the author assumes that dreams offer signs that, if correctly interpreted, will reveal someone’s medical condition.

The question is: why?

The answer to this question combines multiple factors.

Firstly, the Greeks had long treated dreams as something special. Today, we know, thanks to brain-imaging scans, that in a full night’s sleep, normal people will virtually always dream. This might seem striking to us because many people don’t remember their dreams, but we can confirm that our impressions of dreamless nights are mistaken. The Greeks don’t know that, and they think that dreams are much rarer. Ancient people inferred from the rarity that something special was going on, especially in times when someone might vividly recall seeing overflowing rivers or trees shedding all their leaves.

Another reason is that ancient Greeks hated dissection. They were strongly opposed to being around corpses, and the thought of violating one by cutting it opening and studying its insides was anathema even to the most scientifically-minded Greek. By the time of Aristotle (384 - 322 BC), systematic animal dissection was a common practice, from which insights about the interior of the human body were inferred, often (but not always) mistakenly. Hippocratic authors, such as the author of On Regimen IV, sometimes used animal dissection to make inferences about human internal anatomy, but many Hippocratic texts attempt to make such inferences on different grounds.

For instance, they might appeal to dreams.

Dreams were thought to be a convenient way of making claims about human internal anatomy. The author does not intend for dreams to tell us where the organs are or what they look like, but they cut to the chase: they tell us how we can “live a healthy life” (§93). That’s more important than knowing a series of anatomical facts. After all, merely knowing human internal anatomy would not really help a doctor. The crucial things are what’s discussed in this text: how to diagnose someone, and then how to treat them.

This might be a bit surprising because it seems weird that someone would think that dreams could clue us into what is going on in a person’s body, but it makes sense when we consider the author’s view of the soul and its relationship to the body.

The author thinks that the soul is, during the day, kept busy by the body. There’s too much stimulation: the soul’s parts divide their attention between hearing what the body hears, seeing what the body sees, and so on. It never enjoys “any independence,” due to its stimulation when the body is awake (§86). However, when the body sleeps, the soul is free. Free to do what? It’s free to do what the body does when it is awake. When the body sleeps, our soul “sees what is visible, hears what is audible, walks, touches, feels pain, ponders” — and, so, every activity that the body performs with the soul during the day is performed “by the soul during sleep” (§88).

That’s how dreams come to be reliable guides to diagnosing illness, according to the author. Dreams are the product of our soul seeing what is going on in the body. During the daytime, in contrast, our souls attend to what the body sees: the external world and the visible objects that it contains.

In this way, the author of On Regimen IV came to develop a novel and interesting view of how medical practitioners should diagnose diseases.