 This realist stance is stated in the context of examining al-Bitruji’s astronomical proposals. Gersonides’ contention is that “no argument can nullify the reality that is perceived by the senses, for true opinion must follow reality but reality need not conform to opinion” (Goldstein 1974, p. 24). That Gersonides clearly considered his own observations to be the ultimate test of his system is explicit from his attitude towards Ptolemy. The importance of empirical observation cannot be underestimated, he claims, and he values his own observations over those of others. “We did not find among our predecessors from Ptolemy to the present day observations that are helpful for this investigation except our own” (Wars, V.1.3, p. 27), he says in describing his method of collecting astronomical data. Often his observations do not agree with those of Ptolemy, and in those cases he tells us explicitly that he prefers his own. Gersonides lists the many inaccuracies he has found trying to follow Ptolemy’s calculations. Having investigated the positions of the planets, for example, Gersonides encountered “confusion and disorder” which led him to deny several of Ptolemy’s planetary principles (Goldstein 1988, p. 386). He does warn his colleagues, however, to dissent from Ptolemy only after great diligence and scrutiny. Both John Buridan and Gersonides took the first steps in the 14th century toward a principle of inertia; in the words of Touati, Gersonides was the first to “have a presentiment of the principle of inertia” (see Touati 1973, pp. 103, 311). Glasner supports Touati’s contention, claiming that “Gersonides had a rudimentary, not yet correct, but insightful understanding of the concept of inertial motion.” Gersonides was aware of its significance and used it in order to lay the basis for a new physics, an alternative to that of Aristotle. (see Glasner 2015, p 36) He did not rely on a theory of impetus [as did Buridan], which turned out to be an advantage.