The ability to observe is the ability to reliably detect, but that is not all observation is. A thermometer reliably detects temperature yet does not observe the temperature, whereas I do, even though in terms of reliability I cannot match the thermometer. An observation is detection accompanied by active classification and, typically, the subsequent formation of opinion. Even when we say of an animal that it can see something we mean more than that it reliably detects things of a certain sort but also that it deals with such things as objects, gauging their usefulness or lack thereof in relation to its interests and to other aspects of the world. This crucial addendum to the notion of observation can be enshrined in slogan: observers are potential believers. This marks the difference between mere detecting mechanisms and observers.

The contemporary scientific anti-realism of one such as van Fraassen, which is developed in detail under the label of “constructive empiricism” in van Fraassen (1980), can be characterized in terms of detecting mechanisms.