 In a second argument, Carneades points to objects that are similar to one another: can the wise person discern any two eggs, two grains of sand, and so on? The Stoics have multiple replies. It is conceivable that, in some contexts of action, the wise person assents to what is reasonable (eulogon) (DL 1.177), without having a cognitive impression of how things are. Or, if faced with the task to identify grains of sand while lacking a cognitive impression, the wise person can suspend judgment. However, the wise will train themselves so as to be able to perceive minute differences (Cicero, Acad. 2.57), where it might be important to do so. This point is backed up by Stoic physics: no two items in the universe are identical, and their differences are in principle perceptible (Sedley 1982, 2002). Carneades replies that even if no two things were exactly alike (consistent with his general line of argument, he does not take a stance on such questions), a very close similarity could appear to exist for all perceivers (Cicero, Acad. 2.83–5); that is, the impressions of two items, though in fact these items might differ from each other, could be indistinguishable (Nawar 2017, Shogry 2018 and 2021). Discussion continues with a move on the part of the Stoics: they add to their definition of the cognitive impression “one that has no impediment.” Sometimes an impression is—as it were, by itself—cognitive, but is unconvincing due to external circumstances (SE M 7.253). It is a difficult question whether this addition harms the Stoics more than it helps them. If the initial conception of a cognitive impression hangs on the idea that something about its phenomenological nature, or something internal to the impression, marks it as cognitive, the Stoics give up on a crucial assumption if they grant that sometimes there are “impediments.” If, however, cognitive impressions are differentiated by a causal feature (the way they are caused by the ‘imprinter’ which causes the ‘imprint’), the further addition might help (Frede 1983, Nawar 2014), since the impediment might need to be removed before the causal connection could be confirmed (Hankinson 2003).