Xenocrates

Xenocrates of Chalcedon (396 BC to 314 BC) was a Greek philosopher who studied under Plato and later led the Academy in Athens. He entered the Academy about 376 BC, travelled with Plato to Syracuse about 367 BC, and left Athens with Aristotle after Plato died in 347 BC. Called back by Speusippus in 340 BC, he won the election to succeed him as head of the Academy by a few votes, and he refused Athenian citizenship because he disliked the city's ties to Macedonia. Diogenes Laertius lists two mathematical books by him, On numbers and The theory of numbers, both lost. He held that matter is made of indivisible units, and Plutarch reports his attempt to count the syllables that the letters of the alphabet can form, which may be the first combinatorial problem about permutations.