Paracelsus was born near Zurich in 1493. His father was a country physician with an interest in alchemy which he shared with his son, who would later become the first proponent and theorist of iatrochemistry.
Paracelsus left home to study when he was 14, and traveled widely as a student and later as a roaming physician. It is certain that he studied for some time with Johannes Trithemius, and also that he held a high opinion of himself, particularly in comparison with the academics and academic physicians for whom he often expressed a scathing and witty contempt. His abrasive demeanor and rash temper caused him to lose position after position, and stay on the move. He died in 1541 in Salzburg. Though he was well enough known to be a controversial figure, few of his writings were published while he was alive.
After his death, Paracelsus' writings came to be widely published, and a school of sympathetic thinkers coalesced around his work. The Paracelsians extended Paracelsus' antipathy for academia into their articulation of natural philosophy, and rejected the dominant Aristotelian paradigm as antiquated theory constructed of paganism, strange mathematics, and philosophical baling wire. This rejection of Aristotelian thought led quite naturally to their concurrent rejection of Galen. This picture is not held unequivocally by historians, however, some of whom envision Paracelsian thought as a synergy of Galenic theory (appropriated in a kind of hostile take-over, but appropriated rather than rejected nonetheless) and medieval German mysticism. There is textual foundation for this interpretation as well, as it appears that Paracelsus (while firmly committed to integrating Reformationist theology with his science) was actively engaged in alchemical and magical study - and may have even thought of himself as a kind of magus.
Paracelsus' characteristic refusal to participate in neat self-compartmentalization extended to his practice of speaking in scathing terms about Aristotelians and Galenists while retaining some aspects of their philosophy to bulwark his own. Whether or not these acts of concurrent acceptance and rejection constitute a coherent program of response to his ancients and contemporaries is debatable, but it was certainly a common enough trope in his work. For example, his generally negative assessment of mediaeval alchemists sits strangely with his use of Rupescissian and Lullian methods for extracting quintessence, or ethyl alcohol, from various substances.
Paracelsus' philosophy is not always consistent, sometimes contradictory, and challenges the modern reader to read without seeking after for modern coherencies at the expense of medieval coherencies.
Recommended readings:
Paracelsus, Four Treatises
Bruce Moran, The Alchemical World of the German Court: Occult Philosophy and Chemical Medicine in the Circle of Moritz of Hessen