Physicists frequently analyze a physical situation starting at some moment in time, and ignore what happened before that moment in time. To do this, however, they need to understand what was happening at that initial moment. That is, they need Initial Data. This is very similar to using Boundaries, which requires having Boundary Conditions.
Come, you lost Atoms, to your Centre draw,
And be the Eternal Mirror that you saw:
Rays that have wander'd into Darkness wide,
Return and back into your Sun subside.
From Farid al-Din Attar's
The Conference of the Birds