Leonardo Bruni, Letter to Giovanni Marrasio:
The madness of poets, then, stems from the Muses, while the madness of lovers comes from Venus. This arises, however, from the contemplation of true beauty, looking at the image of which we are taken away by the sharpest and most violent of our senses, struck dumb and as though placed outside of ourselves, seized away with all of our senses focused on it. Therefore, it is no less truly than elegantly said that the mind of a lover leads its life in the body of another.
This inflamed occupation and seizure of the soul is called love: a certain divine alienation, or a forgetting of oneself, or a transfusion of one’s being into that whose beauty you admire. If you call this madness and insanity, I will concede and confess it, as long as you understand that no poet is good (nor can a poet be good) unless they be seized by madness of this sort; nor do they see the future when they deliver prophecy, unless it be through this kind of madness, or is God worshiped perfectly and gloriously unless it be through this kind of alienation from one’s mind.
Poetarum ergo furor a Musis est; amantium vero a Venere. Oritur autem hic ex verae pulchritudinis contemplatione, cuius effigiem visu intuentes acerrimo ac violentissimo sensuum nostrorum, stupentes ac velut extra nos positi, totis affectibus in illum corripimur, ut non minus vere quam eleganter dictum sit amantis animam in alieno corpore vitam ducere. Haec igitur vehemens occupatio animi atque correptio amor vocatur: divina quaedam alienatio ac veluti sui ipsius oblivio et in id quoius pulchritudinem admiramur transfusio. Quam si furorem ac vesaniam appellas, concedam etiam atque fatebor, dummodo intelligas neque poetam bonum esse ullum posse nisi huiusmodi furore correptum, neque futura praevidere vaticinantes, nisi per huiusmodi furorem, neque perfecte neque eximie deum coli, nisi per huiusmodi mentis alienationem.