Comments on: Receiving Images

By: Dan Schwartz

Dan Schwartz — Mon, 28 Apr 2008 21:03:12 +0000

One more thing: it’s not clear to me whether Lucretius thinks of everyday objects–like the book on the other side of a room–as shrinking with distance. What I have in mind here is this: in a way, it is true that that book does not look smaller than it actually is. We don’t have to infer the actual size of the book in the way that we have to infer the actual size of the sun; rather, we are directly aware of the actual size of the book, and only as philosophers or mathemaitcians do we come to think about the magnitude of the image itself.

I’m not sure whether any of that is in Lucretius, though. Ideas?

By: Dan Schwartz

Dan Schwartz — Mon, 28 Apr 2008 20:54:47 +0000

Okay, thanks. I’m not sure why I missed that passage.

However, I have to disagree with you when you say that the passage makes Luc. seems slavishly dogmatic. To someone who doesn’t know geometry, or think about its application to optics, the facts here aren’t obvious. I think there is an argument in the passage about the moon:

One premise seems to be that it is the intervening medium which distorts, blurs, and shrinks images as they travel. Another premise–and this one is plausibly derived from observation–is that IF the medium affects images at all, it always blurs them first before it shrinks them. In other words: If images are shrunk, then they are blurred; but if they are blurred, they are not necessarily shrunk. Now, since the moon is not blurred,then, by modus tollens on the first of these two conditionals it follows that the image of the moon is not shrunk. It is roughly of the same size as the actual object.

He has other arguments there which, I grant you, are implausible, but how about this one?

By: Nat

Nat — Sun, 27 Apr 2008 02:18:13 +0000

V.563-91.

And he seems to be aware of Per’s worry in that passage. He says, “For all objects that we view from a considerable distance through a large tract of air become blurred in appearance before their outline is diminished. Consequently, since the moon displays a distinct form and a clearly defined outline, it must appear to us on earth just as it really is on high — with its true contour and in its true dimensions.”
So, he sees a conflict here, but he’s not worried about it. Celestial bodies are exceptions to the general principal of blurring, but Luc. doesn’t tell us why. He just infers from this that our perceptions of the heavens must be veridical in every way.

I love how shameless the Epicurians are on this point. When I suggested that this makes Luc. seem slavishly dogmatic, Monte pointed out that Luc. breaks with Epicurus on the value of poetry. What’s the less plausible commitment:
1. Poetry is vicious
or
2. The sun is the size of a cantaloupe?
eh… a toss up.

By: Dan Schwartz

Dan Schwartz — Sat, 26 Apr 2008 15:14:47 +0000

Nat,

Where does Lucretius say that celestial bodies are exactly the size they appear to be? I recall this in Cicero, but at the time I thought Cicero was giving us an unfair account.

By: voidobsequy

voidobsequy — Fri, 25 Apr 2008 05:04:43 +0000

By the way, I’m Nat.

By: voidobsequy

voidobsequy — Fri, 25 Apr 2008 05:04:21 +0000

I have a few scattered points to make about this passage.
First, and most directly relevant to your question, there is something distinctive about the light from celestial bodies, according to Lucretius. In general, something far away looks tiny (e.g. armies in military practice) but celestial bodies are exactly the size they appear to be. This has something to do with the intensity of the flame (or something; I couldn’t follow the embarrassing rationalization at all). Maybe this characteristic (whatever it is) explains why our perceptions of the heavens isn’t hopelessly blurry.
Come to think of it, is he really committed to us having an especially accurate picture of the heavens? Sure, we get the general layout right and we can see exactly what the sizes of the bodies are, but does he suggest that we can pick out all of the details of their surfaces?

My bigger worry with the reflecting-pool passage is that it doesn’t make any sense. How does that show the speed of images? If the image had to leave a star and make it to our eyes from the moment we put the water in place, then sure that would be some impressive velocity. But that’s clearly not the case. Objects are sloughing off images in all directions all the the time. The image we see left the stars long before the water is in place.
Certainly, the image has to travel the distance from the pool to our eyes in an instant, but this isn’t the distance which Lucretius is so impressed with. “Do you not see now that an image falls instantaneously from the ethereal regions to the regions of the earth?”
He really must think that the image doesn’t leave the star until the water is put in place. But why? And what is distinctive about a reflection, anyway? Why not draw the same conclusion just from looking up at the sky? That is, why doesn’t he think that images must travel from the heavens to our eyes in the moment it takes us to tilt our heads to look up?