The whole prolepses thing is a hard nut to crack.
Yes it is, as is a lot of Epicurean philosophy. As to justice I am totally with you, because I cannot imagine that "justice" or any "virtue" is anything but a thought construct.
I tend to think that we aren't looking deeply enough at the big picture. I think Sedley is right that the issue of how images work and how the mind deals with them is important and has lots of facets. The joke from Cicero to Cassius in January 45 BC about the mind being struck by images seems right on point - and it is interesting that Cassius Longinus denies the aspect that Cicero is alleging:
QuoteDisplay MoreFor it somehow happens, that whenever I write anything to you, you seem to be at my very elbow; and that, not by way of visions of images, as your new friends term them, who believe that even mental visions are conjured up by what Catius calls spectres (for let me remind you that Catius the Insubrian, an Epicurean, who died lately, gives the name of spectres to what the famous Gargettian [Epicurus], and long before that Democritus, called images).
2 But, even supposing that the eye can be struck by these spectres because they run up against it quite of their own accord, how the mind can be so struck is more than I can see. It will be your duty to explain to me, when you arrive here safe and sound, whether the spectre of you is at my command to come up as soon as the whim has taken me to think about you - and not only about you, who always occupy my inmost heart, but suppose I begin thinking about the Isle of Britain, will the image of that wing its way to my consciousness?
[15.19] Cassius to Cicero
[Brundisium, latter half of January, 45 B.C.]
I hope that you are well. I assure you that on this tour of mine there is nothing that gives me more pleasure to do than to write to you; for I seem to be talking and joking with you face to face. And yet that does not come to pass because of those spectres; and, by way of retaliation for that, in my next letter I shall let loose upon you such a rabble of Stoic boors that you will proclaim Catius a true-born Athenian.