TIL that Eye of Newt was just another name for mustard seed.

This is pretty dubious. Follow the trail of sources back on that, and you come to PGM XII.401-44 which you can read here. It is an Egyptian text from sometime between 100 BCE and 400 CE in which describes a code used by temple scribes to obfuscate magical spells so that the general public didn't misuse them. It includes several similar replacements, e.g. "blood of a snake" really means "hematite". It does NOT include in the code any eye of newt or mustard seed or the other Macbeth ingredients. The source is from a collection and translation of Egyptian papyri from 100 BCE and 400 CE that were collected starting in the 1700s. There is no reason to believe that William Shakespear (or 11th century Scottish Whiches for that matter) would have been familiar with the Egyptian temple scribe magical code.

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