What does the law of superposition state

In physics and systems theory, the superposition principle,[1] also known as superposition property, states that, for all linear systems, the net response at a given place and time caused by two or more stimuli is the sum of the responses that would have been caused by each stimulus individually. So that if input A produces response X and input B produces response Y then input (A + B) produces response (X + Y).

The homogeneity and additivity properties together are called the superposition principle. A linear function is one that satisfies the properties of superposition.

your answer is "the 1950's"