The development of Fakhr al-Dīn al-Rāzī’ thought on Avicennian and post-Avicennian r resentationalism and conceptualism is marked by increasing skepticism and critical engagement. Avicenna’s representationalism posits that knowledge is relies on mental forms whose quiddities are identical to those of the objects of knowledge. Some post-Avicennians reformulate this account by viewing mental forms as different from the objects in quiddity (i.e., as mere images). Al-Rāzī is critical of both Avicennian representationalism and representationalism simpliciter. He rejects two foundational premises of Avicennian representationalism—that mental existence is different from concrete existence, that pure quiddity is existentially indifferent—based on the non-equivocity of existence, as well as Avicennian representationalism itself, because it would lead to the impossibility of knowing the extra-mental concomitants of quiddities. Against representationalism simpliciter, al-Rāzī contends that one of its premises—knowledge necessarily requires a specific non-relative accident inhering in the knower—is insufficiently justified, as well as demonstrably false based on his doctrine that knowledge is a pure relation. Avicenna’s conceptualism concerns three classes of things, i.e., non-existent objects of knowledge, definitional parts of simple quiddities, and secondary intelligibles (logical properties). Post-Avicennian conceptualists go beyond the Shaykh, expanding the third class to include properties like existence, unity, thingness, modalities, relations in general. Al-Rāzī’s attack against conceptualism about non-existent objects of knowledge criticizes both the general premise that existence is extensionally unlimited and the specific premise that imaginative objects have extra-mentally non-existent quiddities. Al-Rāzī also elaborates arguments against conceptualism in general, based on correspondence and on the removal of the mind.