Kalvesmaki, Joel. 2013. The Theology of Arithmetic: Number Symbolism in Platonism and Early Christianity. Hellenic Studies Series 59. Washington, DC: Center for Hellenic Studies. http://nrs.harvard.edu/urn-3:hul.ebook:CHS_KalvesmakiJ.The_Theology_of_Arithmetic.2013.
Excursus B. The Pythagorean Symbol of the Τετρακτύς
παγὰν ἀενάου φύσεως ῥιζώματ᾽ ἔχουσαν
No, by the one who grants our head the τετρακτύς,
Fount possessing roots of everlasting nature. 
In light of the authentic fragments of Philolaus, these two lines can be reasonably interpreted to suggest that the ancient Pythagoreans held that the first four numbers were forged out of the principles of nature (in Philolaus, these are ‘limiters’ and ‘unlimiteds’) to provide a “spring” for the physical world. There is the intriguing possibility that the couplet comes from the same literary milieu as Philolaus’ lost work On Nature.