matter cannot be created or destroyed

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Wrongly do the Greeks suppose that aught begins or ceases to be;

for nothing comes into being or is destroyed;

but all is an aggregation or secretion of pre-existing things
so that all becoming might more correctly be called becoming mixed, and all corruption, becoming separate.

–Anaxagoras
circa 450 B.C.

...

We may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment; the quality and quantity of the elements remain precisely the same; and nothing takes place beyond changes and modifications in the combination of these elements. Upon this principle the whole art of performing chemical experiments depends: We must always suppose an exact equality between the elements of the body examined and those of the products of its analysis.

–Antoine-Laurent de Lavoisier
26 August – 8 May 1794

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