In classical physics, the problem of observation was mainly a practical one: how to construct instruments of sufficient accuracy for observing, say, the planets in their courses or a micro-organism in its nutrient fluid. It is then tempting to assume that the particle has an accurate position and momentum all the time, just as in classical physics, although we can never know both at once. Against this, Bohr has stressed that any prediction of what the particle will do next must be based on what we know about it, so that it is a purely academic question whether an observable that has not been measured possesses an unknown but well-defined value. The dyestuff crystals in the polaroid absorb that component of electric vector which is parallel to their orientation and hence perpendicular to axis of the polaroid. The experimenter could use a polarizing device that transmits completely those photons which have a particular elliptic polarization, characterized by two parameters.