Some of Kepler's works seem very different in character. His youthful Mysterium cosmographicum (1596) argues for heliocentrism on the basis of metaphysical, astronomical, astrological, numerological and architectonic principles. By contrast, Astronomia nova (1609) is far more tightly argued on the basis of only a few dynamical principles. In the eyes of many, such a contrast embodies a transition from Renaissance to early modern science. This paper suggests that Karl Popper's fallibilist and piecemeal approach, and especially his theory of errors, might prove extremely helpful in resolving such alleged tension. By abandoning the perspective of the inductivist philosophy of science, which is forced by its own standards to portray Kepler as a “sleepwalker”, the paper focuses on the method he followed: he never hesitated to discuss his own intellectual journey, offering a rational reconstruction of the series of false starts, blind alleys and failures he encountered. The critical dialogue he managed to establish in private correspondence with fellow astronomers he later transplanted into his printed works, whose structure closely resembles that of a dialogue, however implicit.