In pursuit of the unicorn.

This short review was prompted by The Physiological Society's recent online symposium on variability. It does not deal with a specific methodology, but rather with the myth that certain environmentally-induced clinical conditions can be identified, quantified, simplified and monitored with a single methodology. Although this might be possible with some clinical conditions, others resist the prevailing reductionist approach of minimizing rather than exploring variation in pathogenesis and pathology, and will not be understood fully until the variation in cause and effect are embraced. This is likely to require comprehensive methodologies and collaboration.