This personal scientific memoir describes how a chance observation concerning artery disease in children with rare inherited abnormalities of metabolism, often called experiments of Nature, led to the discovery of a new theory of heart disease, cancer and diseases of ageing. As explained in subsequent chapters, this new theory places the amino acid homocysteine at the heart of altered metabolism of diseases of ageing. Not only does this theory illuminate and explain the origin of these diseases, development of this theory logically leads to a novel and revolutionary metabolic and nutritional program for disease prevention and treatment, potentially benefiting individuals and populations.