 The appeal to common sense (or what Valla considers as such) informs his critique of Aristotelian natural philosophy. He insists on commonplace observations and experiences as criteria for testing ideas and hypotheses. On this basis, many of Aristotle’s contentions, so Valla argues, are not true to the facts (Repastinatio, 1:98–112; DD 1:174–99). He rejects or qualifies a number of fundamental tenets of Aristotelian physics, for instance that movement is the cause of heat, that a movement is always caused by another movement, that elements can be transformed into one another, that each has its own proper qualities (heat and dryness for fire, heat and humidity for air, etc.), that there are pure elements, that the combination of heat and humidity is a sufficient condition for the generation of life, and so forth. Valla often uses reductio ad absurdum as an argumentative strategy: if Aristotle’s theory is true, one would expect to observe phenomena quite different from the ones we do, in fact, observe. In arguing, for instance, for the existence of a fiery sphere below the moon, Aristotle had claimed that “leaden missiles shot out by force melt in the air” (De caelo II.7, 289a26–28). Valla rejects this claim by appealing to ordinary experience: we never see balls—whether leaden, iron, or stone, shot out of a sling or a cannon—heat up in the air; nor do the feathers of arrows catch fire. If movement is sufficient to produce heat, the spheres would set the air beneath in motion; but no one has ever observed this. Arguably, the importance of Valla’s polemic here is not so much the quality of his arguments as the critical tendency they reveal, the awareness that Aristotle’s conclusions often do not conform to daily experience. While he does not develop his critique in the direction of an alternative natural philosophy as later Renaissance philosophers such as Bernardino Telesio (1509–1588) and Francesco Patrizi of Cherso (1529–1597) would do, Valla contributed to undermining faith in the exclusive validity of the Aristotelian paradigm.