The future is contingent. It can unfold differently, hinging on chance or choice within the present. This Element tells the story of how these twin concepts have developed across human history. Arcing from our earliest ancestors, through al-Ghazālī, to S. J. Gould, the Element demonstrates how humans realised the future is an undecided, contingent place – at scales leading beyond the biographical, up to the planetary, and beyond. It pinpoints this realisation as an ongoing and unfinished intellectual revolution. Just as the telescope revealed Deep Space in the 1600s, and the geologists' hammer revealed Deep Time in the 1800s, contemporary developments in science are revealing what I call Deep Possibility. This is the realisation that there is far more possible than will ever be actual. It is this that makes history matter, and gives contingency its bite, insofar as it forces acknowledgement that not all outcomes will come to pass regardless.