[The measurement theory and the example of the number zero].

For basic research, measurement used in our medical routine doesn't seem to be self-evident. Classical physics claim and demand that everything should be measured with a maximum of exactness in order to avoid errors principly. Results of measurement and diagnosis are often regarded as on and the same thing. For quantum physics, measuring means rather description, approximation, and even perturbation due to manipulation. In strabismology, zero is the starting point of measurement and defines normality. The examination of 70 patients with asthenopia has reveiled that zero-localisation at the Maddox Wing doesn't imply parallelism of the eyes but an esophoria combined with an ocular-cervical syndrome.