Why is the original rhythm of Gregorian chant difficult to determine?

Determining the original rhythm of Gregorian chant is difficult primarily because the early musical notation used to record these melodies—known as neumes—did not consistently or clearly indicate specific time values for the notes.1

While neumes were effective at helping singers recall the general direction and ornamentation of a memorized melody, they functioned within a semi-oral tradition where the music was largely learned by ear rather than read from a precise score. Because composers of that era did not notate rhythm or tempo with the level of detail found in later Western music, the rhythmic interpretation of these chants remains a subject of ongoing scholarly controversy. It was not until the latter half of the 13th century that musical notes with defined time values began to evolve from these earlier signs, leaving the precise rhythmic intentions of the original Gregorian compositions open to historical debate.12