By Aileen R Das
Associated and sometimes identified with the life-giving (or vital) principle, heat occupied a central place in ancient Greek, and subsequently Roman and medieval Islamicate, theories about the human body and its care. The medical literature surviving from classical Greece shows that early doctors’ understanding of human physiology was greatly informed by philosophical speculations about the basic constituents of the world. Heraclitus of Ephesus (fl. 500 BCE) appears to be the first natural philosopher to give fire a primary role in the cosmos; according to him, everything originates from fire, which undergoes various changes to become the materials that we see around us. His now fragmentary writings do not discuss medical or biological themes, but later ‘Pre-Socratics’ – a modern term that describes Heraclitus and other thinkers before or roughly contemporary with Socrates – such as Empedocles (495–435 BCE) did explain how this element affected the body. 
Both a physician and a philosopher, Empedocles of Akragas is the progenitor of the four element theory, according to which earth, water, fire, and air are the building blocks of the universe, and he asserted that heat was responsible for sexual differentiation. In his philosophical poem On Nature, Empedocles remarks, ‘For in its warmer part the womb brings forth males, and that is why men are dark, more manly, and shaggy’ (fr. 67).
The authors of the Hippocratic corpus developed several of their therapies in light of the notion that an innate heat sustains essential processes in the body such as growth and digestion. The intensity of this heat supposedly varied not only according to sex – with men being warmer than women – but also from person to person. Thus, when deciding on a course of treatment, the doctor had to make sure that they did not excessively increase or reduce the natural heat of their patients. Dietary regimens were the mainstay of Hippocratic therapeutics, for doctors working in this tradition assigned to food a range of properties (cooling, warming, drying, and moistening, to name just a few) that could influence the condition of the body. For example, the Hippocratic treatise Regimen II recommends that the herb coriander, which is described as being ‘hot and astringent’, be eaten to combat heartburn and to induce sleep.
None of the Hippocratic writers offer an overarching theory of the powers of nutriment and other natural substances. Rather, centuries later the physician Galen (d. c. 217 CE) of Pergamum, who drew on the Hippocratics, their philosophical precursors, and earlier pharmacological writers, formulated a system that ranked the properties of plants, minerals, and animal products. The dividing line between what counted as a drug as opposed to a food was blurry in the ancient (as well as medieval) world, so Galen elaborates his theory in both his dietetic and pharmacological works. On the Powers and Mixtures of Simple Drugs, which lists several hundred one-ingredient drugs, offers the most comprehensive account; it relates that all substances possess a mixture of active (hot or cold) or passive qualities (wet or dry) in four varying degrees of intensity, with the first degree being weak and the fourth strongest. For example, in the entry on the chaste-tree (vitex agnus-castus), Galen reports that the leaves and seeds of this Mediterranean plant is warm and dry to the third degree. 
By learning the properties and strengths of a range of materia medica, the doctor can select the appropriate remedy that will match their patient’s imbalance. Regarding the power of the chaste-tree, Galen recommends that the seeds be used to dissolve wind in the stomach and to relieve uterine pain, but he cautions that they are so warming that they can cause a headache. Thus, to avoid this affect, he advises that they be ingested with sweetmeats or other dessert items.
While Galen’s theory of the potency of natural substances was extremely influential throughout antiquity and the middle ages, later medical thinkers looked to redress his failure to explain how a doctor (or pharmacist) calculates the right proportion of ingredients in a multi-ingredient (that is ‘compound’) drug to achieve the desired potency. The Muslim philosopher Abū Yaʿqūb ibn Ishāq al-Kindī (c. 801–66), who was not a doctor himself but had sponsored the Arabic translations of Greek medical works, developed a complex arithmetical theory to quantify the strength of a drug that contained varying degrees of warmth, for instance. According to it, a substance’s intensity increases with an increase in degree according to the double ratio. Thus, if one takes a ‘temperate’ drug that has equal parts of warmth and coldness and doubles the parts of warmth, the drug will be hot in the first degree; if the parts of warmth are quadrupled, then the drug is hot in the second degree and so on. With these proportions in mind, the practitioner can weigh out the simple ingredients of the compound drug to obtain the intended strength. Al-Kindī’s solution to the gap in Galen’s pharmacology was popular not only among medieval Islamicate but also European doctors, who read it through a 12th-century Latin translation.