For in disappointment we are 'outside of' or 'beside' ourselves and, in a word, blinded, so that sometimes, if we have a sponge or a bit of wool in our hands, we pick it up and throw it, as if that would achieve something.How often would people throw sponges about? Were there sponges just lying around in ancient houses like those foam stress-relief toys?  It reminds me of the other very famous instance of someone getting all fed up and throwing a sponge, namely the story of Apelles the painter in Sextus Empiricus PH 1.28:
Once, they say, when Apelles was painting a horse and wished to represent in the painting the horse's foam, he was so unsuccessful that he gave up the attempt and flung at the picture the sponge on which he used to wipe the paints of his brush, and the mark of the sponge produced the effect of a horse's foam (Bury's translation).For Sextus, this is an example of how the sceptic might stumble upon tranquillity about a given question not by finding a convincing answer but by coming to see how all the possible answers are somehow not convincing enough. Best then to suspend judgement and -- hey presto! -- that's something that brings along the tranquillity we wanted all along. Apelles' story was perhaps quite famous (I wonder if it was in Antigonus of Carystus' account of the painter's life; Antigonus also wrote a Life of Pyrrho, the philosophical inspiration for Sextus' scepticism). But anyway, here's my thought:
Is Chrysippus' reference to throwing a sponge in frustration part of a discussion over the Apelles' story, or something like it? Perhaps it goes like this: the Stoics interpret Apelles' frustration in a way consistent with their general intellectualist account of the emotions. At the root of it is some kind of dispositional belief combined with some judgment about the current failings of his attempts to paint the horse's foam. To get things right, for the Stoics, Apelles ought to get his general beliefs right about what truly matters and what does not. Does it really matter if in the end your painting looks a bit crappy? As in their more famous account of the archer, a good painter is a painter who does all a painter can do to try to get the right result. The final result itself if not ethically significant. (This seems to me not altogether plausible, at least in the case of painters, but that's another story. A good painter is surely someone able to produce good paintings and, because in Apelles' case we are thinking about mimetic art, a good painting will be one which convincingly represents the intended subject; the end result in this instance is clearly rather important...)
Anyway, Sextus has a different story: for him, what matters is that Apelles did get the right result after all (the sponge did create the desired effect), but it did not happen in the way Apelles had expected. The frustration he feels at the repeated failure of his individual attempts at the right technique for painting the foam leads cumulatively to a feeling that there is no particular way to get this right. And it is this position of dissatisfaction which leads to his gesture of surrender -- throwing the sponge -- which unexpectedly leads to the right result.
Now, I have no idea how plausible this is. In any case, the story of Apelles is a nice way to contrast the Stoics and sceptics on an important bit of psychology. And, of course, for Sextus, Apelles' story stands for a general account of Pyrrhonist method, not just an account of a Pyrrhonist's emotional life. But I do wonder if buried somewhere in the mass of evidence we have lost there is some connection between Chrysippus' remark and Sextus' story.
 Come to think of it, didn't the Romans -- and probably Greeks -- use sponges as a way of cleaning themselves after using the toilet? And probably also in the bath? You can learn a lot about Greek toilets here. Not sure whether that's a plausible situation for one to feel beside oneself in disappointment, though...