ABSTRACT
M and P now want to incorporate the more familiar dynamical variables into their model: position and momentum. Before building a three-dimensional model, they decide to get their bearings by imagining a one-dimensional world. M starts things off with the following comment: while it might seem reasonable to employ the vector space R as a model of a one-dimensional world, there is a small problem: the vector space has a special point 0, while the physical world presumably has no such distinguished point. For this reason, she proposes the following definition, which amounts to “forgetting the origin” in the vector space R.
