2 Memory Hierarchy Design
Ideally one would desire an indefinitely large memory capacity such that any particular … word would be immediately available. … We are … forced to recognize the possibility of constructing a hierarchy of memories, each of which has greater capacity than the preceding but which is less quickly accessible.
A. W. Burks, H. H. Goldstine, and J. von Neumann
Preliminary Discussion of the Logical Design of an Electronic Computing Instrument (1946)