Varia cogito mecum: Originary and Derivative Certainty of the Multiplicity of Things in the Cartesian Age and in Leibniz

This paper examines how some early modern philosophers, culminating with Leibniz, grappled with a tension internal to Cartesianism: while the cogito grants immediate certainty of one's own existence as a thinking subject, the existence and multiplicity of external things remains epistemically precarious. A trajectory is traced through Descartes, Geulincx, Spinoza, Tschirnhaus, and De Volder, analyzing how each handles the experiential datum expressed by formulas such as varia a me cogitantur (“various things are thought by me”). For Descartes, certainty about external multiplicity is only derivative, grounded in divine veracity. Geulincx radicalizes this by reducing the soul to a mere spectator of the world. Spinoza grounds the perception of plurality in the layered individuation of bodies and their mutual affections. Tschirnhaus elevates consciousness of multiple things to a foundational principle of all human knowledge, placing it alongside the cogito itself. Leibniz goes furthest: he recognizes “Varia a me cogitantur” as a first truth of fact on a par with the Cartesian cogito, binding it to the metaphysical impossibility of solipsism and the real existence of other subjects — a cornerstone of his ‘realistic idealism.’