Alchemy as a proto-science
The common perception of alchemists is that they were pseudo-scientists, crackpots and charlatans, who attempted to turn lead into gold, believed that the universe was composed of the four elements of earth, air, fire, and water, and spent most of their time concocting miraculous remedies, poisons, and magic potions.
Although some alchemists were indeed crackpots and charlatans, most were well-meaning and intelligent scholars; among their number can be counted such distinguished scientists as Sir Isaac Newton. These people in many ways served as innovators, and attempted to explore and investigate the nature of chemical substances and processes. They had to rely on experimentation, traditional know-how, rules of thumb — and speculative thought in their attempts to uncover the mysteries of the physical universe.
At the same time, it was clear to the alchemists that "something" was generally being conserved in chemical processes, even in the most dramatic changes of physical state and appearance; i.e. that substances contained some "principles" that could be hidden under many outer forms, and revealed by proper manipulation. Throughout the history of the discipline, alchemists struggled to understand the nature of these principles, and find some order and sense in the results of their chemical experiments — which were often undermined by impure or poorly characterized reagents, the lack of quantitative measurements, and confusing and inconsistent nomenclature.