Full Idea
By deriving his radiation law, Planck had inadvertently introduced the idea that energy itself could be 'quantized'.
Gist of Idea
Planck introduced the idea that energy can be quantized
Source
Jim Baggott (The Quantum Story: 40 moments [2011], 01)
Book Reference
Baggott,Jim: 'The Quantum Story' [OUP 2016], p.16
A Reaction
He earlier assumed energy is continuously variable. I presume this means that the older idea of energy is now subsumed into the concept of fields, which are quantized into particles. The powers of nature are found in the fields.