The basic empirical procedure is observation. Both measurement and experiment involve observation, whereas the latter is often done without quantitative precision (i.e. without measuring) and without deliberately changing the values of certain variables (i.e. without experimenting). The object of observation is, of course, an actual fact; the outcome of an act of observation is a datum — a singular or an existential proposition expressing some traits of the result of observing. A natural order to follow is, then: fact, observation, and datum. Our discussion will close with an examination of the function of observation in science.