"The Knight-Errant of Philosophy"

That is the epithet assigned to Giordano Bruno by Pierre Bayle, according to this brief account of Bruno's life and thought.

Bruno was born near Naples in 1548 and died in flames in Rome in 1600. In between, he lived a restless life, moving through various Italian cities to Paris, where he interested the king in his famed arts of memory, then to England, where he befriended Sir Philip Sydney, gained the favor of Elizabeth, and published a few key works, including the Heroic Furors, dedicated to Sydney. He visited Oxford while there, and, as he'd done in other locales, he departed in disgust, writing that the Oxford profs "knew more about beer than about Greek."

From England he went to Germany, where he managed to be excommunicated by the Lutherans, then on to Venice, perhaps the most intellectually "open" city of the day. It was there that the Inquisition arrested him and had him extradited to Rome, where he remained imprisoned for 7 years until his execution in the Campo dei Fiori on Feb. 17, 1600. In that Roman square, a statue of the rogue priest/theologian/natural philosopher/magus/satirist/playwright/heretic memorializes the event.

Whatever Bruno's philosophy ultimately has to teach, it's fair to say that it is hauntingly evocative - richly figural and allegorical, tending to meld disciplines and the study of nature, theology, and science as if they were so many elements of a vast Bouillabaisse. Every account attempting to summarize some core of his teaching sounds unlike every other account. The body of Bruno's work, like its earthly equivalent, vanishes within the fires it feeds. Let's hear a bit of what he sounds like (in translation):

It is proof of a base and low mind for one to wish to think with the masses or majority, merely because the majority is the majority. Truth does not change because it is, or is not, believed by a majority of the people.

There is no absolute up or down, as Aristotle taught; no absolute position in space; but the position of a body is relative to that of other bodies. Everywhere there is incessant relative change in position throughout the universe, and the observer is always at the center of things. 

The universe is then one, infinite, immobile.... It is not capable of comprehension and therefore is endless and limitless, and to that extent infinite and indeterminable, and consequently immobile.

In this infinite space is placed our universe (whether by chance, by necessity or by providence I do not now consider). 

Make then your forecasts, my lords Astrologers, with your slavish physicians, by means of those astrolabes with which you seek to discern the fantastic nine moving spheres; in these you finally imprison your own minds, so that you appear to me but as parrots in a cage, while I watch you dancing up and down, turning and hopping within those circles.

My son, I do not say these are foals and those asses, these little monkeys and those great baboons, as you would have me do. As I told you from the first, I regard them [Aristotle; Plato] as earth's heroes. But I do not wish to believe them without cause, nor to accept those propositions whose antitheses (as you must have understood if you are not both blind and deaf) are so compellingly true. 

One can perhaps see in this description how Milton might have recognized a kindred imagination:

Bruno also affirmed that the universe was homogeneous, made up everywhere of the four elements (water, earth, fire, and air), rather than having the stars be composed of a separate quintessence. Essentially, the same physical laws would operate everywhere, although the use of that term is anachronistic. Space and time were both conceived as infinite.. . . 

Under this model, the Sun was simply one more star, and the stars all suns, each with its own planets. Bruno saw a solar system of a sun/star with planets as the fundamental unit of the universe. According to Bruno, infinite God necessarily created an infinite universe, formed of an infinite number of solar systems, separated by vast regions full of Aether, because empty space could not exist. (Bruno did not arrive at the concept of a galaxy.) Comets were part of a synodus ex mundis of stars, and not—as other authors maintained at the time—ephemeral creations, divine instruments, or heavenly messengers. Each comet was a world, a permanent celestial body, formed of the four elements. 

Clearly, Bruno was no Girolamo Savonarola (1452-1498). Indeed, had Bruno lived during the time of that rigorous Dominican, it's likely that his furious books, along with his hide, would have fed the Bonfire of the Vanities. It's equally likely he'd have said to Savonarola what he reportedly told the judges who ordered him to be burned:

Perchance you who pronounce my sentence are in greater fear than I who receive it.