He was interested in philosophy and was an expert on the art of memory; he wrote books on mnemonic technique, which Frances Yates contends may have been disguised Hermetic tracts. The writings attributed to Hermes Trismegistus were, in Bruno's time, recently rediscovered and at that time were thought to date uniformly to the earliest days of ancient Egypt. They are now believed to date mostly from about 300 A.D. and to be associated with Neoplatonism. Bruno embraced a sort of pantheistic hylozoism, rather than orthodox Christian Trinitarian belief.

Bruno was also heavily influenced by the ideas of Copernicus and by the newly rediscovered ideas of Plato as well as the teachings ascribed to Hermes Trismegistus[1]. Other influences included Thomas Aquinas, Averroes, Duns Scotus, Marsilio Ficino, and Nicholas of Cusa.

In 1576 he left Naples to avoid the attention of the Inquisition. He left Rome for the same reason and abandoned the Dominican order. He travelled to Geneva and briefly joined the Calvinists, before he was excommunicated, ostensibily for his adherence to Copernicanism and left for France.

In 1579 he arrived in Toulouse, where he briefly had a teaching position. At this time, he began to gain fame for his rodigious memory. Bruno's feats of memory were apparently based, at least in part, on an elaborate system of mnemonics, but many of his contemporaries found it easier to attribute them to magical powers.

For seven years, he enjoyed the protection of powerful French patrons, including Henry III. During this period, he published 20 books, including several on memory training, Cena de le Ceneri (The Ash Wednesday Supper, 1584), and De l'Infinito, Universo e Mondi (On the Infinite Universe and Worlds, 1584). In Cena de le Ceneri he defended the theories of Copernicus, albeit rather poorly. In De l'Infinito, Universo e Mondi, he argued that the stars we see at night were just like our Sun, that the universe was infinite, with a "Plurality of Worlds", and that all were inhabited by intelligent beings (see the Drake equation). These two works are jointly known as his "Italian dialogues." In 1582, Bruno penned a play summarizing some of his cosmological positions, titled Il Candelaio ("The Torchbearer").

Travel years
In 1583, he went to England with letters of recommendation from Henry III. There he sought a teaching position at Oxford, but appears to have given offense and was denied a position there (and elsewhere in England).

In 1585 he returned to Paris. However, his 120 theses against Aristotelian natural science and his pamphlet against the Catholic mathematician Fabrizio Mordente soon put him in ill favor. In 1586, following a violent quarrel about "a scientific instrument", he left France for Germany.

In Germany he failed to obtain a teaching position at Marburg, but was granted permission to teach at Wittenberg, where he lectured on Aristotle for two years. However, with a change of intellectual climate there, he was no longer welcome, and went in 1588 to Prague, where he obtained 300 taler from Rudolf II, but no teaching position. He went on to serve briefly as a professor in Helmstedt, but had to flee again when he was excommunicated by the Lutherans, continuing the pattern of Bruno's gaining favor from lay authorities before falling foul of the ecclesiastics of whatever hue.

1591 found him in Frankfurt. Apparently, during the Frankfurt Book Fair, he received an invitation to Venice from one Zuane Mocenigo, who wished to be instructed in the art of memory, and also heard of a vacant chair in mathematics at the University of Padua. Apparently believing that the Inquisition might have lost some of its impetus, he returned to Italy.

He went first to Padua, where he taught briefly, but the chair he sought went instead to Galileo Galilei, so he went to the University of Venice. He briefly functioned as a tutor to Mocenigo, who may have been disappointed that Bruno was merely teaching him a complex system of mnemonics rather than some form of magic. When Bruno attempted to leave Venice, Mocenigo denounced him to the Inquisition. He was arrested May 22, 1592, and given a first trial hearing before being sent for trial in Rome in 1593.

Trial and death
In Rome he was imprisoned for six years before he was tried, lastly in the Tower of Nona. He tried in vain to obtain a personal audience with Pope Clement VIII, hoping to make peace with the Church through a partial recantation. His trial, when it finally occurred, was overseen by the inquisitor, Cardinal Saint Robert Bellarmine, who demanded a full recantation, which Bruno refused. Consequently, he was declared a heretic, handed over to secular authorities on January 8, 1600. At his trial, he said: "Perhaps you, my judges, pronounce this sentence against me with greater fear than I receive it." A month or so later he was brought to the Campo de' Fiori, a central Roman market square, his tongue in a gag, hung upside-down naked and burned at the stake, on February 17, 1600.

Since 1889, there has been a monument to Bruno on the site of his execution, erected by Italian Masonic circles.

Although the actual charge against Bruno was docetism, (adherence to the doctrine that Jesus did not actually have a physical body and that his physical presence was an illusion), and despite the fact that his theoretical work cannot be considered scientific, some authors have claimed Bruno as a "martyr of science". They see a parallel between his persecution and the Galileo affair, asserting that even though, unlike Galileo, Bruno's theological beliefs were a factor in his heresy trial, Bruno's Copernicanism was also a factor.

But the above "connection" may be exaggerated, or even plainly false. For example: according to the Stanford Encyclopedia of Philosophy, "…in 1600 there was no official Catholic position on the Copernican system, and it was certainly not a heresy. When…Bruno…was burned at the stake as a heretic, it had nothing to do with his writings in support of Copernican cosmology."[2] In fact, the precise charges of heresy on which Bruno was finally condemned are unknown, as the official record has long been lost. The role (if any) of his heliocentric teachings and belief in an infinite universe is not a matter that can be conclusively proved on either side.

All his works were placed on the Index Librorum Prohibitorum in 1603. Four hundred years after his execution, official expression of "profound sorrow" and acknowledgement of error at Bruno's condemnation to death was made, during the papacy of John Paul II. Attempts were made by a group of professors in the Catholic Theological Faculty at Naples, led by the Nolan Domenico Sorrentino to obtain a full rehabilitation from the Catholic authorities. However, while the Church was able to express regret for the violent death of a man, it could not condone the activities of a man who was, as the London embassy period shows, not in favour of any sort of religious tolerance and brought about the deaths of Roman Catholics.

The cosmology of Bruno's time
In the second half of the 16th century, the theories of Copernicus began diffusing through Europe. Although Bruno did not wholly embrace Copernicus's preference for mathematics over speculation, he advocated the Copernican view that the earth was not the center of the universe, and extrapolated some consequences which were radical departures from the cosmology of the time.

According to Bruno, Copernicus's theories contradicted the view of a celestial sphere, immutable, incorruptible, and superior to the terrestrial region. Bruno went beyond the heliocentric model to envision a universe which, like that of Plotinus in the third century A.D., or like Blaise Pascal's nearly a century after Bruno, had its center everywhere and its circumference nowhere.

Few astronomers of Bruno's generation accepted even Copernicus's heliocentric model. Among those who did were the Germans Michael Maestlin (1550-1631), and Cristoph Rothmann and the Englishman Thomas Digges, author of A Perfit Description of the Caelestial Orbes (sic). Galileo (1564-1642) and Johannes Kepler (1571-1630) were younger, so they do not figure at this time. Bruno himself was not an astronomer, but one of the first to embrace Copernicanism as a world view, rejecting geocentrism. In works published between 1584 and 1591, Bruno enthusiastically supported Copernicanism.

According to Aristotle and Plato, the universe was a finite sphere. Its ultimate limit was the primum mobile, whose diurnal rotation was conferred upon it by a transcendental God, not part of the universe, a motionless prime mover and first cause. The fixed stars were part of this celestial sphere, all at the same fixed distance from the immobile earth at the center of the sphere. Ptolemy had numbered these at 1,022, grouped into 48 constellations. The planets were each fixed to a transparent sphere.

Copernicus conserved the idea of planets fixed to solid spheres, but considered the apparent motion of the stars to be an actual motion of the earth; he also preserved the notion of an immobile center, but it was the Sun rather than the Earth. He expressed no opinion as to whether the stars were at a uniform distance on a fixed sphere or scattered through an infinite universe.

Bruno's cosmology
Bruno believed, as is now universally accepted, that the Earth revolves and that the apparent diurnal rotation of the heavens is an illusion caused by the rotation of the Earth around its axis. He also saw no reason to believe that the stellar region was finite, or that all stars were equidistant from a single center of the universe. In these respects, his views were similar to those of Thomas Digges in his A Perfit Description of the Caelestial Orbes (1576).

However, Digges considered the infinite region beyond the stars to be the home of God, the angels, and of the holy. He conserved the Ptolemaic notion of the planetary spheres, considered Earth the only possible realm of life and death, and a unique place of imperfection and change, compared against the perfect and changeless heavens.

In 1584, Bruno published two important philosophical dialogues, in which he argued against the planetary spheres. (Two years later, Rothmann did the same in 1586, as did Tycho Brahe in 1587.) Bruno's infinite universe was filled with a substance -- a "pure air", aether, or spiritus -- that offered no resistance to the heavenly bodies which, in Bruno's view, rather than being fixed, moved under their own impetus. Most dramatically, he completely abandoned the idea of a hierarchical universe. The Earth was just one more heavenly body, as was the Sun. God had no particular relation to one part of the infinite universe more than any other. God, according to Bruno, was precisely as present on Earth as in the Heavens, an immanent God rather than a remote heavenly deity.

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. There was no room in his stable and permanent universe for the Christian notions of divine Creation and Last Judgement.

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 sustained at the time -- ephemeral creations, divine instruments, or heavenly messengers. Each comet was a world, a permanent celestial body, formed of the four elements.

Bruno's cosmology is marked by infinitude, homogeneity, and isotropy, with planetary systems distributed evenly throughout. Matter follows an active animistic principle: it is intelligent and discontinuous in structure, made up of discrete atoms. This animism (and a corresponding disdain for mathematics as a means to understanding) is the most dramatic respect in which Bruno's cosmology differs from what today passes for a common-sense picture of the universe.