Filling out space – The Ether and the Dispositions of Matter in Kant’s Opus Postumum

For Kant, a body fills out space by means of its causal efficacy. The essential properties of matter are hence dependent on underlying forces and it is one task of the Opus postumum (OP) to reconstruct the system of forces. In order to avoid an infinite regress of causal explanations, this system of forces needs to account for a primitive origin of all mechanical moving forces in something that is constitutive of forces, yet radically different - this is the ether that fills all space that constantly moves or vibrates all the parts of the material bodies internally. I argue that we can read Kant’s argument for the existence of the ether through the lens of dispositions: essential properties of matter, such as ponderability, coercibility, cohesibility, and exhaustibility, should be understood as dispositions. These dispositions are ‘physically conditioned,’ as Kant calls it, on the ether. To display their manifest and perceivable properties, the ether acts as the ubiquitous and universal ‘activation stimulus’ of the dispositions that make up material bodies. He causes them to constantly display their manifest properties that make them continuously perceivable and causally efficient. Thereby Kant offers a solution to a problem formulated by Simon Blackburn, namely that purely dispositional matter would lead us into skepticism.