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This article aims to clarify the concept of occult quality used by Pomponazzi to give a physical explanation of the unusual effects of nature (so called « miracles »). When he relates these qualities to the notion of « specific form », Pomponazzi brings up an ancient debate about Avicenna’s theory of formal causality, opposing St Thomas to the consensus of philosophers and physicians, namely to Pietro d’Abano’s Conciliator. In this way, he dissociates physical causality from the consideration of the essence and substantial form, and gives an unusual significance to the « equivocal » causality of matter. The De incantationibus can be regarded as a major source of the later materialistic interpretation of the Aristotelian form.