What evidence makes it difficult to know which ideas came directly from Pythagoras?

Distinguishing the original teachings of Pythagoras from those of his followers is difficult due to several factors:

• Lack of Contemporary Records: Pythagoras likely wrote no books, and the first accounts of his life appeared roughly 150 years after his death.1
• Esoteric Transmission: Early doctrines were passed down orally and through cryptic akousmata (orally transmitted sayings), which disciples later interpreted according to their own convictions.23
• Attribution of Authority: Pythagoreans consistently supported their own doctrines by indiscriminately citing their master’s authority, leading to the misattribution of later discoveries—such as the Pythagorean theorem—to Pythagoras himself.1
• Biased Literary Sources: Surviving evidence is often filtered through later authors, such as Plato’s pupils or Aristotelians, who frequently reinterpreted Pythagorean concepts to fit their own philosophical agendas.23
• School Evolution: The school lacked a uniform system, and its split in the 5th century BCE, combined with the accumulation of legendary traditions, created significant confusion regarding the original intellectual core.23