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Canguilhem’s book, The Normal and the Pathological, written in the middle of the twentieth century, remains the landmark investigation into the concept of normality.⁵ Canguilhem was a French medical doctor, historian and philosopher of science. His study concerns the technical question of medical normality. This ostensibly specialist work is more generally illuminating than one might imagine, not least because medicine has operated as a motor for the diffusion of the concept of normality throughout our culture.

Modern medicine is based on a notion of medical normality, which is to say on defining health as accordance with a predefined norm. Canguilhem’s key question is that of the origin of this norm. Modern medicine clearly considers itself to be a scientific enterprise, based on empirical study and objective criteria. Its basic norms do not really measure up to this self-image, however.

Canguilhem notes that it is commonly believed in the medical profession that what is normal is simply what is average, such that the normal condition of health can be defined by observing what most people’s condition is. He concludes, however, that medical normality cannot possibly be defined or derived in this way. To be normal is defined in medicine as being optimally healthy, and this is clearly not the average condition of human beings. Rather, most people are to some extent, in some way or other, unhealthy, which is to say that they deviate from the norm. Unless exactly as many people deviate in two opposite directions – for example, exactly as many people have high blood pressure as have low blood pressure, to the same extent – then the average will not be the norm. And such an absolutely symmetrical deviation in both directions from the norm never occurs in reality.

In fact, the word ‘normal’ only came to be associated with averages when statisticians in the late nineteenth century applied this word – which was by that time already in use in other technical fields, and in medicine in particular – to an extant statistical idea that they had called by other names previously, dubbing this now the ‘normal distribution’.⁶ However, I will argue that this invention of a statistical notion of the normal thenceforth serves to give a patina of objectivity to the concept of normality in general.

Canguilhem concludes that the medical notion of normality, although presented as an objective and scientific judgement, is in fact normative, which is to say, a judgement of what things should be like. Such normative judgements cannot be empirical inferences from scientific study – as Ian Hacking points out, statistical judgements of normality cannot have any normative implications⁷ – but rather must be a priori principles. These principles are, in a word, norms, imaginary standards of perfection to which reality is held.

Although it might seem like a great scandal that medicine is based on prejudices, Canguilhem does not reject these norms. He is instead quite clear that modern medicine needs the yardstick of normality to operate. Discarding it would cost untold lives. He instead suggests reforming the standard of normality by abandoning the pretence that it is objective and acknowledging its subjectivity by making patients themselves the final arbiters of whether they actually are sick.

For my part, I am not advocating even this: unlike Canguilhem, I do not have a medical background, and am not directly concerned with medical norms. Rather, I am concerned with what has happened as the notion of normality has exploded out of medicine and become a social phenomenon over the last two centuries.