“The third problem  [the two first ones being (i) infinite-dimensional space and (ii) preparation of a system in an arbitrary state] seems potentially more tractable, although not yet solved. This is the problem of observation. Indeed, we may distinguish two problems of observation: ignorance and error. By “ignorance” I mean that we look at an infinite-dimensional state, but record only a small number of parameters. Even if we measure without error, the data describe only a point in a finite-dimensional space. Thus, the observation procedure, at best, defines a projection map from the infinite-dimensional state space, S, to a record space, R, of finite dimension. I shall refer to this as the output projection map.”

Ralph H. Abraham