Browsing some Aristotle today I came across this in De Partibus Animalium 3.6. He is discussing the lungs and making a case that they are for the sake of respiration. As part of the discussion he dismisses the idea that lungs are there to cope with the ‘jumping’ of the heart. His argument for this is interesting because it seems to offer some evidence for his distinction between human and animal psychological capacities (669a17-23):
τὸ δὲ πρὸς τὴν ἅλσιν εἶναι τὸν πλεύμονα τῆς καρδίας οὐκ εἴρηται καλῶς· ἐν ἀνθρώπῳ τε γὰρ συμβαίνει μόνον ὡς εἰπεῖν τὸ τῆς πηδήσεως διὰ τὸ μόνον ἐν ἐλπίδι γίνεσθαι καὶ προσδοκίᾳ τοῦ μέλλοντος, ἀπέχει τ’ἐν τοῖς πλείστοις πολὺν τόπον καὶ κεῖται τὴν θέσιν ἀνωτέρω τοῦ πλεύμονος, ὥστε μηδὲν συμβάλλεσθαι τὸν πλεύμονα πρὸς τὴν ἅλσιν τῆς καρδίας.
Here is a translation by William Ogle (1912):
It has been said that the lung exists as a provision to meet the jumping (halsis) of the heart. But this is out of the question. For man is practically the only animal whose heart presents this phenomenon of jumping, inasmuch as he alone is influenced by hope and anticipation of the future. Moreover, in most animals the lung is separated from the heart by a considerable interval and lies above it, so that it can contribute nothing to mitigate any jumping.
See Hippoc. Morb. Sacr. 17 (§XX in the Loeb) for the idea that the phrenes jump and cause palpitations as a result of unexpected excessive pleasure or distress (εἴ τι ὥνθρωπος ὑπερχαρείη ἐξ ἀδοκήτου ἢ ἀνιηθείη, πηδῶσι [αἱ φρένες] καὶ ἅλσιν παρέχουσιν).
This is the bit that interests me: only humans have hope (elpis) or expectation for the future (prosdokia); this is why only humans experience their heart ‘jumping’. This suggests that hope and expectation here are being understood to have some rational component or source. What exactly distinguishes them from the kinds of forethought that non-rational animals are capable of?