He was noted for intellectual audacity and self-confidence and his reliance on his own ability was shown, among other things, in the request he is supposed to have made to Cleanthes: "Give me the principles, and I will find the proofs myself."
Han var kendt for intellektuelt vovemod og selvtillid og hans tillid til sig selv viste sig bl.a. ved at han skal have sagt til Kleanthes "Giv mig principperne og jeg finder selv beviserne."