The law of definite proportions might seem obvious to the modern chemist, inherent in the very definition of a chemical compound. At the end of the 18th century, however, when the concept of a chemical compound had not yet been fully developed, the law was novel. In fact, when first proposed, it was a controversial statement and was opposed by other chemists, most notably Proust's fellow Frenchman Claude Louis Berthollet, who argued that the elements could combine in any proportion. The very existence of this debate underscores that at the time, the distinction between pure chemical compounds and mixtures had not yet been fully developed.
The law of definite proportions contributed to, and was placed on a firm theoretical basis by, the atomic theory that John Dalton promoted beginning in 1803, which explained matter as consisting of discrete atoms, that there was one type of atom for each element, and that the compounds were made of combinations of different types of atoms in fixed proportions.
It may be noted that although very useful in the foundation of modern chemistry, the laws of definite proportions is not universally true. There exist non-stoichiometric compounds whose elemental composition can vary from sample to sample. An example is the iron oxide wüstite, which can contain between 0.83 and 0.95 iron atoms for every oxygen atom, and thus contain anywhere between 23% and 25% oxygen. In general, Proust's measurements were not accurate enough to detect such variations.
In addition, the isotopic composition of an element can vary depending on its source, hence its weight in a pure stoichiometric compound may vary. This fact is used in geochemical dating since astronomical, atmospheric, oceanic, crustal and deep Earth processes may concentrate lighter or heavier isotopes preferentially. With the exception of hydrogen and its isotopes, the effect is usually small but measurable with modern instrumentation. An additional note: many natural polymers vary in composition (for instance RNA, proteins, carbohydrates) even when "pure". Polymers are generally not considered "pure chemical compounds" except when their molecular weight is uniform (monodisperse) and their stoichiometry is constant. In this unusual case, they still may violate the law due to isotopic variations.