I have taken back the Philosophy of Knowing course from Ashley. Wasn't too keen on it, but there it is, and I am taking the opportunity to re-read Insight, and Verbum. The first impression is that both texts seem easier and more enlightening than ever before. Of course the class seems much slower, much less interested, not as bright as before. Yesterday I took 'higher viewpoints'. The maths did not appeal to them at all. Or perhaps I was too messy with my boardwork.
This morning I go on to 'inverse insights.' I remember how fascinated I was with this. The lack of intelligibility in uniform motion: Newton's first law, that a body keeps moving in a straight line unless acted upon by an external force. The idea that, if nothing else intervenes, it would keep moving to eternity, no need of energy, fuel, or whatever. The idea that motion does not need explanation, but acceleration. Physics then becomes a theory of accelerations rather than motions. The whole shift from Aristotelian to Newtonian physics. The setting free of investigation, and the consequent flowering of physics.
The other example - which I think I am still digesting - is the question about the trajectory of a coin falling to the floor in a moving train. (I wonder where I got that one.) What is that trajectory? It depends on the framework. W.r.t. the train, it is a perpendicular to the floor; whether the train is moving or not does not matter, provided the motion is uniform motion. From outside the train, it is a diagonal or a curve forward: very interestingly, the coin moves along with the train, even when it is itself 'free' of the hand or of the floor! From outside the earth, the trajectory would be even more complicated; and so on, from outside the solar system....
I think this exemplifies another aspect of the unintelligibility of uniform motion. No laws of physics are affected as long as the framework is in uniform motion / rest. Or: it does not matter whether the framework is in uniform motion or at rest.
Einstein's thought experiment might illustrate this better: two spaceships with no other reference point except each other. Moving at the same speed. Are they moving? Are they at rest? The point is: it does not matter. Uniform motion is relevant only w.r.t. some framework. Otherwise it does not matter.