Full Idea
The idea that electromagnetic waves could be a reality in themselves, independent of any bodies, did at that time not occur to the physicists.
Gist of Idea
It was formerly assumed that electromagnetic waves could not be a reality in themselves
Source
Werner Heisenberg (Physics and Philosophy [1958], 07)
Book Reference
Heisenberg,Werner: 'Physics and Philosophy' [Penguin 1989], p.99
A Reaction
'At that time' is when they thought the waves must travel through something, called the 'ether'.