Galen gives a few directions how to regulate medical observation and experience, although they cannot be said to carry us very far on the road to modern laboratory research. He saw the value of “long experience,” a phrase which he often employs.[723] He states that one experience is enough to learn how to prepare a drug, but to learn to know the best medicines in each kind and in different places many experiences are required.[724] Medicinal simples should be frequently inspected, “since the knowledge of things perceived by the senses is strengthened by careful examination.”[725] Galen advises the student of medicine to study herbs, trees, and fruit as they grow, to find out when it is best to pluck them, how to preserve them, and so on. But elsewhere he states that it is possible to estimate the general virtue of the simple from one or two experiences.[726] However, he suggests that their effect be noted in the three cases of a perfectly healthy person, a slightly ailing patient, and a really sick man.[727] In the last case one should further note their varying effects as the disease is marked by any excess of heat, cold, dryness, or moisture. Care should be taken that the simples themselves are pure and free from any admixture of a foreign substance.[728] “It is also essential to test the relation to the nature of the patient of all those things of which great use is made in the medical art.”[729] One condition to be observed in experimental investigation of critical days is to count no cases where any slip has been made by physician or patient or bystanders or where any other foreign factor has done harm.[730] Galen was acquainted with physical experiments in siphoning, for he says that, if one withdraws the air from a vessel containing sand and water, the sand will follow before the water, which is the heavier (_sic?_).[731] [Sidenote: Difficulty of medical experiment.] Galen also points out some of the difficulties of medical experimentation. One is the extreme unlikelihood of ever being able to observe in even two cases the same combination of symptoms and circumstances.[732] The other is the danger to the life of the patient from rash experimenting.[733] Thus Galen more than once tells us of abstaining from testing some remedy because he had others of whose effects he was surer. [Sidenote: Empirical remedies.]