The adjective ‘potential’ sets a logical trap. A potential x is not a kind of x, but at best a thing of a different kind that is capable of becoming an x (so, for example, the destruction of a potential x is not the same as the destruction of an actual x). In Aristotelian terms, potential is a power to change into different states. In biology the nature of this power remained a central question until the discovery of ‘instructions’ for development in the shape of the DNA molecule. The difference between potentiality and actuality is also one of the puzzling questions raised by quantum mechanics, according to which a particle such as an electron or photon is completely described by a set of potentialities with different probabilities of being realized, until the moment of measurement, when just one of them is recognized as actual. See Schrödinger’s cat.