Constructivism across Islamo-European Boundaries in the Fifteenth Century: The Philosophical Transmission of the Foundations of Qushji’s Astronomical Models

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Constructivism across Islamo-European Boundaries in the Fifteenth Century: The Philosophical Transmission of the Foundations of Qushji’s Astronomical Models

Date Presented:

12–16 June


Qushji, who headed the Samarkand Observatory and was later associated with the scientific community of Constantinople under the Ottomans, wrote his magnum opus, which covers the philosophy of astronomy, as a commentary on Tusi’s Tajrid al-Kalam (Abstraction of Kalam). Qushji’s “moving eccentric” model for Mercury and Venus contained the critical philosophical breakthrough to a mathematical foundation for Earth’s motion. It was specifically this model that F. Jamil Ragep (2005) demonstrated an exact resemblance for with a diagram that appeared in Book XII (Proposition 2) of Regiomontanus’ Epitome of the Almagest (1496). Owen Gingerich (2008), Michael Shank (2008), and others have asserted the same transmission account. Earlier, Swerdlow (1973) remarked that Copernicus relied in both his Commentariolus and his De Revolutionibus upon this eccentric model of the second anomaly for the inferior planets as they appeared in the Epitome. Given the similarity in mathematical proof between that of the inferior planets (which Ptolemy rejected) and that of the superior ones (which he accepted), the breakthrough, therefore, is not one of mathematical proficiency: rather, it is a conceptual one. Ptolemy had disallowed the eccentric equivalence for the inferior planets, because the planets are never in opposition to the Sun, which means that to move the eccentric, the movement would be attributed to the Earth rather than the planet, which of course Ptolemy refused. But Qushji allowed for the possibility of Earth’s motion, as we see in his commentary on the Tajrid; thus, he is the only known pre-Copernican astronomer to have explicated and justified this model. To the contrary, Copernicus used this model to claim his heliocentric theory as a mathematical corollary, which in effect introduces a necessitarian worldview upon a constructive model that was first produced by Qushji based on the contingency of the natural world. By adopting a heliocentric view, Copernicus was applying Aristotelian syllogism to a transmitted foundation of astronomy, and, consequently, resorted to reversing Aristotle’s “stationary Earth” claim to a “stationary Sun.” Qushji never advocated heliocentricism, since constructivism as a theory of necessary knowledge does not lead to it. In fact, both geocentricism and heliocentricism are scientifically untenable, in a strict sense. With constructivism, mental existence allows for universal concepts in the mind, such as constructive mathematical models, to act as an aid to the process of individuation and astronomical perception. Qushji, who made significant progress in constructive semantics (`ilm al-wad`), brought over his general method of constructivism and in particular his individuation theory from his treatises on constructive semantics into kalam’s various philosophical investigations on subjective conceptualism and its diverse empirical applications; extending with that the constructivist theory of knowledge from linguistic usage to mathematical modelling as a bearer of a constructivist philosophy that is diffusive across cultural boundaries, which we later unsurprisingly see resemblance for with Kant’s own Copernican Revolution and beyond.

Last updated on 05/29/2017