It is unwise, though common practice, to base decisions on numerical values without knowing how they can be justified as correctly describing aspects of reality. Scientists have great responsibility in this regard, especially in the social sciences where it is easy to provide an appearance of rigour through the use of numbers and mathematical apparatus. We must be prepared to justify any numbers we use by clarifying the relationship between a numerical variable and a “real thing”. The article defines fundamental, explicit, scientific measurements and how they may be used in a meaningful way. It then considers instruments, in particular pointer measurements, that simulate these fundamental measurements. It concludes with a look at the validity of the resulting information.