Sir Archibald Garrod's concept of chemical individuality introduced a century ago provided the basis for the entire discipline of inborn errors of metabolism. Human disorders are defined by variation in disease-causing mutations, environmental influences, genetic factors other than the primary genetic defect, and evolution itself. Myriad examples support the prescience of Garrod with respect to these issues, each of which has therapeutic implications. Just as Garrod predicted that the future of biochemical genetics would be intertwined with the concept of chemical variability, we might forecast that variation will influence emotions, dreams, and the human thinking process itself.