Stylus Phantasticus was first described by Athanasius Kircher (1601 - 1680) in his publication on music, Musurgia Universalis (1650). Kircher defines the style with this words:
“The fantastic style is suitable for instruments. It is the most free and unrestrained method of composing; it is bound to nothing, neither to word nor to a melodic subject; it was instituted to display genius and to teach the hidden design of harmony and the ingenious composition of harmonic phrases and fugues; it is divided into those [pieces] that are commonly called fantasias, ricercatas, toccatas, sonatas.”
In the end of 16th century, italian composers such as for example Gesualdo (1560 - 1613) and Macque (1548 - 1614), wrote pieces called Stravaganza ('extravagance' or 'fantastic eccentricity'). Stravaganzas followed no specific form and displayed extraordinary melodies, rhythms and harmonies. These features became part of the baroque Stylus Phantasticus.
Early pieces in this style are Fantasias and Toccats for keyboard instruments by composers like Merulo (1533 - 1604) and Frescobaldi (1583 - 1643). Fantasias and Toccatas alternate slow melodic passages with fast and improvisatory passages. We also find this character of freedom, improvisation, virtuosity and imagination in early violin sonatas by for example Marini (1594 - 1663) and Uccellini (1603 - 1680). With Froberger (1616 - 1667), student of Frescobaldi, the style travelled north to Austria. With other travelling musicians the Stylus Phantasticus came to northern Germany where Buxtehude (1637 - 1707) excels in the style.
By the time Mattheson (1681-1764) is discussing the style in his Der Vollkommene Capellmeister (1739), the emphasis had changed from the composer's freedom to the performer's freedom to improvise. Mattheson says that Stylus Phantasticus is an improvisatory style:
“a singing or playing that comes of the free genius...”
“now swift, now hesitating, now in one voice, now in many voices, now for a while behind the beat, without measure of sound, but not without the intent to please, to overtake and to astonish.”
If we combine the views of Kircher and Mattheson we see that Stylus Phantasticus is music composed in the freedom of fantasy and preformed with the goal to move the passions of the listener.
Dietrich Becker (1623-1679) - Sonata a 2 (Erst Theil, Zwey-stimmiger Sonaten und Suiten, Hamburg, 1674)
Dieterich Buxtehude (1637-1707) - Toccata in G major (BuxWV 165)
Johann Heinrich Schmelzer (c1620-1680) - Sonata IX (Duodena Selectarum Sonatarum, Nuremberg, 1659)
Anonymous (17th century) - Sonata, viola da gamba solo (source: GB-Ob/MS.Mus.Sch.D249)
Dieterich Buxtehude - Sonata III (BuxWV 261, op.2, Hamburg, 1696)
Heinrich Ignaz Franz von Biber (1644-1704) - Sonata V (Sonatæ violino solo, Nuremberg, 1681)
Nicolaus Adam Strungk (1640-1700) - Capriccio, primi tuoni (source: US-NH/Ma.21.Y2.L8(LM5056))
Antonio Bertali (1605-1669) - Sonata a 2 in d minor (source: S-Uu/imhs 1:5)
Philipp Heinrich Erlebach (1657-1714) - Sonata prima (VI Sonate, Nuremberg, 1694)
Dieterich Buxtehude - Sonata a 2 in a minor (BuxWV 272, source: S-Uu/imhs 13:26)