The mutable signs are:
Gemini: – between the spring and summer seasons.
Virgo: – between the summer and autumn seasons.
Sagittarius: – between the autumn and winter seasons.
Pisces: – between the winter and spring seasons.
In a Byzantine scholium to Chapter 2 of the Introduction to astrology by fourth-century Hellenistic astrologer Paulus Alexandrinus, the following clear definition can be found:
“A double-bodied zoidion [sign] is said to be between two seasons, such as Gemini between spring and summer, ending the spring and beginning the summer [...] That is to say, double-bodied as being between the two bodies of spring and summer.”
900 years later, when medieval Italian Guido Bonatti wrote his Liber Astronomiae, in the final years of the thirteenth century, the definition remained the same and his is more verbose:
“The moveable (cardinal) signs are so-called [...] because at the time when the Sun enters them the disposition of the air is changed [...] The common signs are so-called because when the Sun enters any of these signs it makes the time common, neither truly fixed nor truly movable, but it partakes of both, fixed and moveable. Whence part of that time it is of one [nature] and part of the other [...] when [the Sun] leaves Leo and enters Virgo, then the season is changed, and is made partly summer and partly autumnal.”
“The Signs [...] are divided into moveable, fixed and common, [...] [Common] Signs are constituted between moveable and fixed, and retain a property of nature, partaking both with the preceding and consequent Sign [...] They are called bi-corporeal or double bodied, because they represent two bodies: as Gemini (twins), Pisces (two fish).”
The seasonal connection had become more tenuous, although it was doubtless still understood.
Lilly goes on to say that mutable signs are inherently “unstable, and of no resolution, and mutable, perverted, wavering [...] inconstant.” This is a rather dramatic overstatement, but Lilly is trying to create the most striking comparisons he can between the three classes of signs.[original research?]
Modern astrology does tend to regard mutable signs as more unstable and wavering, less strong-willed, than either cardinal or fixed signs, but also more adaptable and can deal more easily with change.
Definition courtesy Wikipedia.