Inside 3-space, we identify otherness as counterforce to will. Specifically, otherness is present as mass. But this presentation is dependent on the sensation of pressure that is a counterforce to will.
Mass combines with extension and impenetrability to inform our notion of substance. Mass is what distinguishes the predicate substance from every other objective predicate. In the end it is effort that forms the empirical basis for our notion of what mass is.
Mass is what gives way on Salvia. The idea that pressure was ever sufficient to establish a truly trans-temporal counterpoint to the incessant variation of inner sense just evaporates. What is left to distinguish subject from object is nothing. Center-self and circumference-object become indistinguishable. All sensation is passing, including the sensation of pressure-against which denotes the noumenal other.
When we schematize the innards of matter, what do we imagine is there? What is the quality of the internal-to-matter? Surely, we take the core of matter to be more than a static extensional keep-out (impenetrability). It is also one that dynamically pushes back. The counterforce of mass is an excellent candidate for bonafide otherness, but even this is just a quale.