 Dalton’s theory gives expression to the idea of the real presence of elements in compounds. He believed that atoms survive chemical change, which underwrites the claim that elements are actually present in compounds. He assumed that atoms of the same element are alike in their weight. On the assumption that atoms combine with the atoms of other elements in fixed ratios, Dalton claimed to explain why, when elements combine, they do so with fixed proportions between their weights. He also introduced the law of multiple proportions, according to which the elements in distinct compounds of the same elements stand in simple proportions. He argued that this principle was also explained by his atomic theory.