You have to bear in mind that until the 17th century or so, the philosophical doctrine that "nature abhors a vacuum" seemed to be backed by very reasonable evidence: namely, the fact that it was basically impossible to actually create a vacuum through any technological means available. And until we had basic theories of gravity and the properties of gases, there was no theoretical justification for how a vacuum could form naturally.

So it makes sense that to most thinkers of the time, it was more plausible for space to be completely filled with some kind of perfectly transparent but substantial aether.

You raise a very interesting point, though. I would be very interested to learn what ancient philosophers thought about atmospheric haze. Did they recognize that the bluish appearance of distant mountains and the reddish appearance of sunrises/sunsets had a common cause? And if so, did they associate it with "air" as an elemental substance, or something else?

> the fact that it was basically impossible to actually create a vacuum through any technological means available

Do you have more sources on this? I would like to read up. As far as I'm concerned, vacuum is trivial to produce: by putting your palms together and then pulling them apart, by using blacksmith's bellows, and a lot of devices that used leather and/or bladders. I think the problem is in recognizing that you created vacuum, rather than creating it, so I would like to know if you have more insight into why you said that.

> by using blacksmith's bellows

Creating a pressure differential where air flows in immediately. As vacuum seemingly cannot be sustained without constant work, it seems that nature does indeed not like vacuums.

> vacuum is trivial to produce

But not trivial to sustain. And it would be a sustained vacuum if it were between the earth and the moon or sun.

you can sustain it if it's properly sealed, but I see your point, people would have a hard time understanding why the outer space would apparently sustain such a large amount of vacuum. It seems unnatural to us, because we're so used to atmospheric pressure that we're taking it for granted.

I also thought of several examples; breathing in/out of a paper bag, hot air balloon, or that experiment where you put heat up or metal container and put the opening on water and it gets sucked in (or crushes a thin can). I can see that it takes work to sustain these, but what about just putting a glass bottle in your mouth and pulling a vacuum and carefully putting your thumb on top. You can create a slight vacuum in a glass bottle this way and it will stay there until you move your thumb and hear the "whoosh". But I guess it still "wants" to fill the void as soon as you unplug it, so I guess I understand.

That kind of effect was studied and used in antiquity, as pneumatics. Aristotle's point of view did not hold sway everywhere or for all of that period; afaik pneumatics started with Empedocles and the clepsydra, and obviously the atomists favored the vacuum.

I've kept out of this conversation because I'm too ignorant, but it seems worth saying that while "it doesn't fit with Aristotle's scheme of the world" may be a good enough explanation for most premodern western thinking, it's hardly all of it.

How does breathing in and out of a paper bag create vacuum? As soon as you inhale, the atmospheric pressure collapses the bag. Bellows are different because they can hold their shape: if you seal all the ends and then expand the bellows, you will have vacuum and the 1atm of pressure trying hard to stop you from doing that.

The same with hot air balloons, there is no vacuum at play there, unless I'm miss-understanding what you're saying.

Here vacuum does not mean low pressure, it means lack of substance.

If you use a strong pump to depressurize and seal a glass bottle you still get a bottle with air inside; you can easily see that there is air inside because if you submerge it in water or oil and break the seal you will see air bubbles coming out.

If you were to try the same with a glass bottle full of water, you will find that is is much much harder to pump the water out than is it to pump the air out.

This is perfectly compatible with the impossibility of an actual vacuum.

The experiment that subverted this view had to use mercury to be practical at the time[1]

In this experiment you essentially use the weight of mercury to pump mercury out of a glass bottle and since air cannot traverse mercury nor glass [2] then it must truly be a full vacuum. [3]

[1] https://en.wikipedia.org/wiki/Torricelli%27s_experiment

[2] This is false, mercury evaporates, quantum tunneling exists, gasses likely can be dissolved in mercury and material are porous.

[3] In some sense you could say that the photons in the bottle make it not a vacuum, as they have mass or also that particles spontaneously appear from a base energy noise or a lot of other oppositions to that being a real vacuum. Still it is definitely a different thing from just low pressure air.

This is the key take away. As a modern human you obviously reason “how they hell could there NOT be vacuum?” , but in those times you had islands of knowledge (no email, mail if you are lucky) and you always reason from what you know at that point.

Just like future readers might shake their head “how did they not know warp drives work like this?”