In 1985, Richard S. Westfall re-proposed the thesis that Galileo ‘stole’ the discovery of Venus’ phases from his pupil, Benedetto Castelli. Westfall believed that Galileo’s observations of Venus were prompted by a letter from Castelli, possibly received by Galileo on 11 December 1610. In this paper, on the basis of my mathematical reconstruction of the entire cycle of Venus’ phases during the second half of 1610, I argue that Galileo’s accurate description of the non-linear nature of the cycle of Venus’ phases – contained in his answer to Castelli and in an almost identical letter to Clavius – could not possibly have been guessed by him subsequent to his receiving Castelli’s letter. Thus, counter to Westfall’s thesis, I conclude that Galileo had really observed Venus over a period of time before receiving Castelli’s letter and therefore did not dishonestly appropriate the merit of the discovery of Venus’ phases.