We had a very good discussion of prolepsis in our Wednesday Zoom tonight and one thing (of several) that come from it is that I definitely think part of the ground work we want to lay is to refer to the case of Centaurs, discussed in Lucretius, and observe that Lucretius said both (1) that the images of the centaur do strike our minds, but also (2) centaurs do not and cannot exist.
I think that gives us some important leads to follow as to what is involved in the faculty of prolepsis, and how we have to distinguish the fact that prolepses are true in the sense of "honestly reported" but not true to the facts in the sense of the conclusion "centaurs exist."
[722] Come now, let me tell you what things stir the mind, and learn in a few words whence come the things which come into the understanding.
[724] First of all I say this, that many idols of things wander about in many ways in all directions on every side, fine idols, which easily become linked with one another in the air, when they come across one another’s path, like spider’s web and gold leaf. For indeed these idols are far finer in their texture than those which fill the eyes and arouse sight, since these pierce through the pores of the body and awake the fine nature of the mind within, and arouse its sensation.
[732] And so we see Centaurs and the limbs of Scyllas, and the dog-faces of Cerberus and idols of those who have met death, and whose bones are held in the embrace of earth; since idols of every kind are borne everywhere, some which are created of their own accord even in the air, some which depart in each case from diverse things, and those again which are made and put together from the shapes of these. For in truth the image of the Centaur comes not from a living thing, since there never was the nature of such a living creature, but when by chance the images of man and horse have met, they cling together readily at once, as we have said ere now, because of their subtle nature and fine fabric. All other things of this kind are fashioned in the same way. And when they move nimbly with exceeding lightness, as I have shown ere now, any one such subtle image stirs their mind; for the mind is fine and of itself wondrous nimble.
[749] That these things come to pass as I tell, you may easily learn from this. Inasmuch as the one is like the other, what we see with the mind, and what we see with the eyes, they must needs be created in like manner.
[752] Now, therefore, since I have shown that I see a lion maybe, by means of idols, which severally stir the eyes, we may know that the mind is moved in like manner, in that it sees a lion and all else neither more nor less than the eyes, except that it sees finer idols.
[757] And when sleep has relaxed the limbs, the understanding of the mind is for no other cause awake, but that these same idols stir our minds then, as when we are awake, insomuch that we seem surely to behold even one who has quitted life, and is holden by death and the earth. This nature constrains to come to pass just because all the senses of the body are checked and at rest throughout the limbs, nor can they refute the falsehood by true facts. Moreover, the memory lies at rest, and is torpid in slumber, nor does it argue against us that he, whom the understanding believes that it beholds alive, has long ago won to death and doom.
[768] For the rest, it is not wonderful that the idols should move and toss their arms and their other limbs in rhythmic time. For it comes to pass that the image in sleep seems to do this; inasmuch as when the first image passes away and then another comes to birth in a different posture, the former seems then to have changed its gesture. And indeed we must suppose that this comes to pass in quick process: so great is the speed, so great the store of things, so great, in any one instant that we can perceive, the abundance of the little parts of images, whereby the supply may be continued.
[777] And in these matters many questions are asked, and there are many things we must make clear, if we wish to set forth the truth plainly.
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So the image of a Centaur is real, and reported honestly to us through the faculty that receives images, and yet we do not conclude simply from perceiving an image of a Centaur that Centaurs are real, because we have all sorts of other observations that through deductive reasoning establish to us, and lead us to conclude, that Centaurs cannot exist, and thus that the images of Centaurs we received were not generated by real Centaurs.
An obvious question arises: If we are to conclude that the gods exist and are blessed and imperishable, it would seem that those conclusions must be based on more observations, and more deductive reasoning, and not simply on the receipt of images. Images constitute real evidence, since they are canonical, and yet we override that evidence by comparing it to other evidence and deem the image evidence insufficient to conclude "centaurs exist in reality."
In the case of gods, what additional observations and reasoning provide the impetus to conclude that gods are blessed and imperishable? We'll no doubt want to look at what else Velleius says for that evidence, including isonomia and infinity and the examples of deductive reasoning that Velleius gives.