John von Neumann: «When we talk mathematics, we may be discussing a secondary language ...»

«When we talk mathematics, we may be discussing a secondary language, built on the primary language truly used by the central nervous system. 

Thus the outward forms of our mathematics are not absolutely relevant from the point of view of evaluating what the mathematical or logical language truly used by the central nervous system is. However, the above remarks about reliability and logical and arithmetical depth prove that whatever the system is, it cannot fail to differ considerably from what we consciously and explicitly consider as mathematics.»