vis a tergo

Like the Evangelists, like Saint Luke, the arist is not a free agent obeying only his own will. His situation is rigidly bound by a chain of prior events. The chain is invisible to him, and it limits his motion. He is not aware of it as a chain, but only as vis a tergo, as the force of events behind him. The conditions imposed by these prior events require of him either that he follow obediently in the path of tradition, or that he rebel against the tradition. In either case, his decision is not a free one: it is dictated by prior events of which he senses only dimly and indirectly the overpowering urgency, and by his own congenital peculiarities of temperament.

Excerpted from The Shape of Time by George Kubler, pg. 50