De vulgari eloquentia (I, iii, 1-3)

(1) Cum igitur homo, non nature instinctu, sed ratione moveatur, et ipsa ratio vel circa discretionem vel circa iudicium vel circa electionem diversificetur in singulis, adeo ut fere quilibet sua propria specie videatur gaudere, per proprios actus vel passiones, ut brutum animal, neminem alium intelligere oppinamur; nec per spiritualem speculationem, ut angelum, alterum alterum introire contingit, cum grossitie atque opacitate mortalis corporis humanus spiritus sit obtentus. (1) Since, therefore, human beings are moved not by their natural instinct but by reason, and since that reason takes diverse forms in individuals, according to their capacity for discrimination, judgement, or choice - to the point where it appears that almost everyone enjoys the existence of a unique species - I hold that we can never understand the actions or feelings of others by reference to our own, as the baser animals can. Nor is it given to us to enter into each other's minds by means of spiritual reflection [speculationem], as the angels do, because the human spirit is so weighed down by the heaviness and density of the mortal body.
(2) Oportuit ergo genus humanum ad comunicandum inter se conceptiones suas aliquod rationale signum et sensuale habere; quia, cum de ratione accipere habeat et in rationem portare, rationale esse oportuit; cumque de una ratione in aliam nichil deferri possit nisi per medium sensuale, sensuale esse oportuit; quare, si tantum rationale esset, pertransire non posset; si tantum sensuale, nec a ratione accipere, nec in rationem deponere potuisset. (2) So it was necessary that the human race, in order for its members to communicate their conceptions among themselves, should have some signal based on reason and perception. Since this signal needed to receive its content from reason and convey it back there, it had to be rational; but, since nothing can be conveyed from one reasoning mind to another except by means perceptible to the senses, it had also to be based on perception. For, if it were purely rational, it could not make its journey; if purely perceptible, it could neither derive anything from reason nor deliver anything to it.
(3) Hoc equidem signum est ipsum subiectum nobile de quo loquimur: nam sensuale quid est, in quantum sonus est; rationale vero, in quantum aliquid significare videtur ad placitum. (3) This signal, then, is the noble foundation that I am discussing; for it is perceptible, in that it is a sound, and yet also rational, in that this sound, according to convention, is taken to mean something.