The possibility of spatially extended unbound singlet states has considerable historical and even philosophical importance, since considering such states eventually led to experimental exploration and verification of what is now called quantum entanglement. Quantum entanglement is the ability of quantum systems to maintain relationships that appear to violate the principle of locality, which Albert Einstein considered fundamental and defended throughout his life. Along with Podolsky and Rosen, Einstein proposed the EPR paradox thought experiment to help define his concerns with the non-locality of spatially distributed singlets, using it as a way to assert that quantum mechanics was incomplete.