Complex sentence: De Finetti developed his ideas on subjective probability in the 1920s independently of Frank P. Ramsey but (according to the preface of his Theory of Probability) he drew on ideas of Harold Jeffreys, I. J Good and B.O. Koopman.
Split into simpler sentences: De Finetti developed his ideas on subjective probability in the 1920s independently of Frank P. Ramsey. Still, according to the preface of his Theory of Probability, he drew on ideas of Harold Jeffreys, I. J Good and B.O. Koopman.