The paper starts by reviewing Imaam Shafi’es original theses on qiyaas in the book al risaalat and looks at them from a modern scientific perspective. It then uses medical examples to discuss the pillars and methods of qiyaas, arkaan al qiyaas; the difference between ‘illat and hikmat in the usage of qiyaas; the classification of qiyaas; the basis for considering qiyaas as a source of legal rulings, hujjiyat al qiyaas; and the methods of qiyaas, manahij al qiyaas.  The paper shows how qiyaas parallels the modern empirical methodology used in medical research. It the proposes a thesis that scientific medical knowledge can deepen the understanding of ‘illat and lead to more relevant and effective rulings based on qiyaas. This is illustrated by discussion of use of qiyaas in the specific case of prohibiting tobacco nicotine. The paper argues that the analogy of tobacco to an intoxicant, khamr, is a legally stronger reason for its prohibition than its association with lung cancer and other human diseases. Nicotine is found in all parts of the tobacco plant and constitutes 5 percent of its weight. It is an alkaloid that is the chief addictive constituent of tobacco. It has a biphasic effect. In small amounts it is a stimulant and in bigger amounts it is a tranquilizer. It is strongly addictive making it difficult for smokers to give up the habit. Nicotine is classifiable as an intoxicant because of its effects on the mind since the Law defines an intoxicant as any substance that clouds the mind, al kahmr ma khaamara al ‘aql. Every intoxicant is forbidden, kullu khamr haraam. What intoxicates in big amounts is also prohibited if taken in small amounts, ma khaamara kathiiruhu fa qaliiluhu haram. The current prohibitions of tobacco are based mainly on very strong scientific evidence at the level of ghalabat al dhann that tobacco causes lung cancer. However some chronic smokers do not get lung cancer because of other mitigating factors such as eating fruits and green vegetables. On the other hand the scientific evidence for stimulant, tranquilizing, and addictive effects of nicotine is at the level of yaqiin because these effects uniformly and always follow nicotine use.