Ideally one would desire an infinitely 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 that 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)
(Quote found in Computer Architecture : A Quantitative Approach 4th Ed. by Hennessy & Patterson)