The square is spiritually charged. It is the product of mankind, it is not copied from nature like the circle.
Notes from Wikiquote
from his interview in the Dutch newspaper N.R.C., 1991; as cited by Din Pieters in 'Raveel: het vierkant als onbeschreven blad', in N.R.C.-online, 5 Jan. 1996 (translation: Fons Heijnsbroek)