Ibn Sina says,
“The state of the finger-nails should be noticed. Softness or dryness of the nails, not due to an extraneous agent, informs one of the state of the temperament.
These qualities are not in themselves a sufficient criterion. There must be signs of balance between heat and cold.
For (a) heat, by its resolving effect, would modify hardness and roughness of feel, and make the patient seem to be attempered and his nature seem soft and moist.
Or, (b) cold—i.e. the opposite—by reason of the great congelation and inspissation it induces, would make the softness of feel in an attempered person seem hard, and give the impression that his nature was dry. For instance, take snow and the sun. Snow congeals and causes coagulation ; the sun causes aggregation of particles. Many persons with a cold, temperament are soft to the feel, and also spare in habit owing to the presence of much crudity in them.”