selection, the prevalence of bacteria resistant to several antibiotics. However, the delay and expense that would be required to perform definitive species identification in every single clinical case are not affordable, so some degree of trade-off is accepted on the principle of the benefits outweighing the risk. Earlier senses of the term Another now-dated sense of the term empiric therapy involves quackery, and empiric as a noun has been used as a synonym of quack. This sense applies when the amount of guessing involved by the clinician transcends so far beyond science that the standard of care is not upheld. Whereas prescribing a broad-spectrum antibiotic to fight a clinically apparent infection as early as possible is entirely prudent and scientific despite the absence of confirmatory cultures, prescribing magic rituals or pseudoscientific schemes is not scientific. The fact that "acting on practical experience in the absence of theory or complete knowledge" can have both legitimate and illegitimate forms stretches back to long before science existed. For example, in the era of ancient Greece, when medical science as we now know it did not yet exist, all medicine was unscientific and traditional; theories of etiology, pathogenetic mechanism, and therapeutic mechanism of action were based on religious, mythologic, or cosmologic ideas. For example, humorism could dictate that bloodletting was indicated for a certain disorder because a supposed excess of water could be rebalanced. However, because such theories involved a great deal of fanciful notions, their safety and efficacy could be slim to negative. In the example of bloodletting to correct excess water, the fact that fluid balance is a legitimate physiologic concern didn't mean that the then-state-of-the-art "understanding" of causation was well founded overall. In this environment where mainstream medicine was unscientific, a school of thought arose in which theory would be ignored and only practical results would be considered. This was the original introduction of empiricism into medicine, long before medical science would greatly extend it. However, by the late 19th and early 20th centuries, as biological and medical science developed, the situation had reversed: because the state of the art in medicine was now scientific medicine, those physicians who ignored all etiologic theory in favor of only their own experience were now increasingly quackish, even though in the era of religion-based or mythology-based medicine (the era of medicine men) they might have been, as viewed through today's hindsight, admirably rational and