Mach, Ernst - What is an atom?
Type of spiritual experience
Such a short interesting little phrase.
The commonly accepted meaning of this is that as we cannot perceive an atom with our 5 senses then it has and may always be a concept of our thought - almost a philosophical concept and not a physical concept.
But there is also the meaning, which I believe to be intentionally meant as well, that the atom itself is a thing of 'thought', made up of the same stuff as thought - that is spirit.
A description of the experience
Atoms cannot be perceived with the senses they are things of thought.