From what I understand, Einstein’s education and cultural milieu was very influenced by Kant, but Einstein himself was personally more interested in Schopenhauer.
To quote Schopenhauer (English translation):
```
But space and time are not only, each for itself, presupposed by matter, but a union of the two constitutes its essence, for this, as we have seen, consists in action, i.e., in causation
```
That's the kind of thinking that could help Einstein formulate an idea of "spacetime"
