Paracelsus thought the Alkahest was an undiscovered element from which all other elements (earth, fire, water, air) were simply derivative forms. Paracelsus believed that this element was, in fact, the philosopher's stone.
In Renaissance alchemy, alkahest was the theorized "universal solvent". It was supposed to be capable of dissolving any composite substance, including gold, without altering or destroying its fundamental components.
Something that can dissolve and break down any substance into its parts in a pure way