Abraham Maslow introduced his now-famous hierarchy of needs in 1943, proposing that people had to achieve a sequential set of conditions to attain complete fulfilment. The five levels of the hierarchy were represented as a pyramid to convey the idea that higher levels, such as self-actualization, could not be reached until lower levels, such as basic physiological needs, were first conquered.

Factoryjoe (CC BY-SA 3.0)