Einstein had become tired of assumptions. He had no particular
objection to the "ether" theory beyond the fact that this "ether"
did not come within the range of our senses; it could not be
"observed." "The consistent fulfilment of the two postulates--'action
by contact' and causal relationship between only such things
as lie within the realm of observation [see Note 2] combined
together is, I believe, the mainspring of Einstein's method of
investigation...." (Prof. Freundlich).