four rivers of heaven
Many religions tote some sort of afterlife: a place you can "live on" in after you die physically, here on earth. Often, there's two places: one where you go when you're good and one where you go when you're bad. In Islam, we have both of those places: heaven (جنّة) and hell (جهنم). We may cover hell a little bit later on but for now, we'll stick to the other place. Heaven is described in many verses of the Quran, from entering one of its hundred of gates (Chapter Thunder) to the types of plants (Chapter Inevitable) and even the dinner plans (same chapter). Because of the immense description of the physicality of heaven, many people believe it to be a real place. But that explanation wasn't good enough for the Nizari mathematician: Muhammad ibn Muhammad ibn Hasan Tusi.
One of the descriptions of heaven comes from Chapter Muhammad:
فِيهَا أَنْهَارٌ مِّن مَّاءٍ غَيْرِ آسِنٍ وَأَنْهَارٌ مِّن لَّبَنٍ لَّمْ يَتَغَيَّرْ طَعْمُهُ وَأَنْهَارٌ مِّنْ خَمْرٍ لَّذَّةٍ لِّلشَّارِبِينَ وَأَنْهَارٌ مِّنْ عَسَلٍ مُّصَفًّى ۖ وَلَهُمْ فِيهَا مِن كُلِّ الثَّمَرَاتِ وَمَغْفِرَةٌ مِّن رَّبِّهِمْ
...wherein are rivers of water unaltered, rivers of milk the taste of which never changes, rivers of wine delicious to those who drink, and rivers of purified honey, in which they will have from all [kinds of] fruits and forgiveness from their Lord...
Quran 47:15 (Shahih International translation) source
The four rivers of heaven are often equated with four actual rivers: the Nile, the Euphrates, Amu Darya, and the Syr Darya. While these four rivers don't intersect on Earth, they're said to come together in heaven. Side note: in some Christian philosophies, the Garden of Eden is similarly placed at the intersection of four rivers, but the rivers converged on Earth, in an area which is now covered by the Persian Gulf.
However, the heavenly versions of these rivers don't flow with just water. One river is of pure (possibly distilled) water. Another is of milk that never goes bad. A third is of honey, and the last is of delicious wine. With the descriptions provided, you can imagine drinking from the rivers, just as one would drink from earthly rivers. But as Tusi explains, it can be a different kind of taste.
aspects of the knowledge
For Nasir adDin Tusi, the rivers of paradise are not physical rivers at all. As a scientist, Tusi at least had an understanding of basic river mechanics: understanding that rivers don't flow with milk, honey, wine, or even water that's super clean. Nor do rivers in Central Asia converge with rivers in Africa. Instead, like many Isma'ili and Sufi philosophers (some of whom he studied with), Tusi sought to explain the metaphor contained in the Quran, as well as it's applicability to humans across time.
Tusi explains the four rivers as four planes (or levels) of knowledge and that drinking from each river would be engaging in a certain sector of knowledge. For Tusi, this revolves around spiritual knowledge, continuing the idea that the human intellect is inextricably linked with divinity. Tusi's levels of knowledge are also closely linked to the Nizari imams (this work was probably written during the time of Imam 'Ala adDin Muhammad). These imams are commonly thought to hold divine wisdom, passed down through heredity. And for Tusi, the higher levels of knowledge grew closer to the knowledge held by the imam.
While Tusi explains the metaphor in terms of exclusivity, we'll also expound on his ideas.
The first river of paradise flows with water, a ubiquitous substance. 71% of the Earth's surface is covered in water. Around 60% of the adult human body is water. It literally falls from the sky; it's water. But what's important for Tusi's explanation is that everything drinks water. Humans, of course, drink water but so do the animals. And so do plants. No living organism on Earth (at least the ones known about in Tusi's time) can survive without water.
For Tusi, this level is knowledge that is accessible to everyone. In scientific terms, these are simply biological processes. Something like cell reproduction isn't often considered to be an intellectual pursuit; most of the time we don't even think about it. But it's something that we share with all organic matter, but also separates us and (according to Tusi) makes us better than things like rocks, sand, dust, and inorganic matter. This can be considered a level of intelligence that only organisms hold, even if literally all organisms hold it. If we're looking at a human perspective alone, this level of knowledge would be for all humans.
Milk, for Tusi, is a little bit more exclusive than water. While water is consumed by every living thing, milk only feeds mammals. Milk isn't produced by lizards, or birds, or fish, nor is it consumed by them. But for mammals, the production of milk is a natural process. Mothers start producing milk while they're pregnant, and babies have an instinctual urge to find and drink it.
In modern terms, this level of knowledge could be considered the limbic system: fight, flight, food, and the other one. These aren't traits shared by all living things, but they serve as a foundation for self-preservation, and an early stage of what can be considered "intelligence". Just like milk encourages us to grow, this level of knowledge sets the foundation for intellectual growth. In the human perspective, this would be people who can think critically, and engage with their surroundings.
Honey is the first substance on this list to be produced through what could be considered labour. Even though the nectar is digested by the bees (a natural process), and the honey is formed through evaporation (another natural process), we can see that there's intent behind this process by the way the bees treat the finished honey (source). Also, very few animals actually eat honey: including humans, bears, and of course, honey badgers. But even then, it takes a great deal of effort to get the honey from the hive. Honey is exclusive in that it takes a certain amount of ingenuity to even get it.
For Tusi, this is the beginning of the intellect. The "honey" level of knowledge is for those that can not think and consider, but also reap the rewards of doing so. These are people that are conscious of the world around him, that have deep understandings of the ways of the universe and can start to grasp at universal truths. Nasir adDin Tusi considered Nizaris and Sufis to have this level of knowledge because of their proximity to the imam.
The wine is of particular note because, in many Muslim traditions, wine is expressly forbidden. The Quran forbids "khamr" (الْخَمْرُ) as a product of Satan (source) and the river here flows with "khamrin" (خَمْرٍ), which comes from the same root. The root "kh-m-r"(خمر)commonly translates to "covering" or "hiding" and in this case means "covering one's ability to think" and "khamr" is often translated as "liquor". (The same root is also used to describe clothing that covers the body completely). Interpretations vary, when it comes to heavenly wine: some say wine made by God doesn't take your inhibitions, others say you can't get drunk because you no longer have a physical form, others say that the prescriptions in the Quran only apply to those living on Earth. Nasir adDin Tusi has a wholly other explanation.
In Tusi's metaphor, wine was the most exclusive of these substances: only humans can produce it (and it's a lengthy process) and only humans drink it. Wine is excluded from all other forms of life.
For Tusi, the wine of heaven had nothing to do with a physical sensation, although the intoxication is a very important aspect. In Tusi's understanding, intoxication means a separation from the physical world. Those who drink (or take other intoxicating substances) often do so to "forget about their problems". For Tusi, these problems were physical restrictions preventing someone from reaching the divine knowledge. With this metaphorical wine, one could relieve themselves of thoughts from the physical world and embrace spirituality wholly. But, just like making wine is a process, so is reaching this level of intelligence. Because it has no terrestrial bounds, it's nearly impossible to reach this level of knowledge in your life. But it's not impossible. In these esoteric beliefs, it's possible to reach this level through intense meditation. However, only the imams and the prophets (according to Tusi) have this level of intelligence inherently. And while many can work toward it, very few people can actually reach it.