"Denis the Carthusian... is the most complete type of religious enthusiast at the end of the Middle Ages. His mental range and many-sided energy are hardly conceivable. To mystic transports, ferocious asceticism, continual visions and revekations he unites immense activity as a theological writer. His works fill forty-five quarto volumes....
He never knew repose. Every day he recites the Psalter almost entirely, and, at any rate, half. He prays continually, while dressing or while engaged in any other occupation. When others go to sleep again after matins, he remains awake. Big and strong, he exposes his body with impunity to all kinds of privations... He feeds, for choice, on tainted victuals.
The enormous amount of theological meditation and speculation which he achieved was not the fruit of a peaceful and balanced life of study; it was carried out in the midst of intense emotions and violent shocks. Visions and revelations are with him ordinary experiences. Ecstasies come to him on all sorts of occasions, especially when he hears music, sometimes in the midst of noble company who are listening to his wise advice.... He is a stammerer. He sees the room of a dying woman full of demons who knock the stick out of his hand. He constantly converses with the dead. When asked if he often sees apparitions of deceased persons, he answers: "Yes, hundreds of times."
Although constantly occupied with his supernatural experiences, he does not like to speak about them, and is ashamed of the ecstasies which earned him among the laudatory surnames of the great theologians that of Doctor Ecstaticus."
- Heuzinga, ibid.