Historically, the understanding of '1 + 1' evolved from concrete counting to abstract representation. Early humans likely began by observing the combination of physical objects. Ancient civilizations developed various numeral systems (e.g., Egyptian hieroglyphs, Roman numerals, Babylonian cuneiform) to represent quantities, and implicitly, their sums. The development of positional notation, particularly with the introduction of zero and the decimal system by Indian mathematicians, dramatically simplified arithmetic operations. It wasn't until the 19th and 20th centuries, with figures like Giuseppe Peano, Gottlob Frege, and Bertrand Russell, that the logical and axiomatic foundations of arithmetic were fully formalized, elevating '1 + 1 = 2' from an intuitive truth to a rigorously proven theorem.