Now, when a person is setting off to discuss the relation between electricity and light, it is very natural and very proper to pull him up short with the two questions: "What do you mean by electricity?" and "What do you mean by light?" These two questions I intend to try briefly to answer. And here let me observe that in answering these fundamental questions, I do not necessarily assume a fundamental ignorance on your part of these two agents, but rather the contrary; and must beg you to remember that if I repeat well-known and simple experiments before you, it is for the purpose of directing attention to their real meaning and significance, not to their obvious and superficial characteristics; in the same way that I might repeat the exceedingly familiar experiment of dropping a stone to the earth if we were going to define what we meant by gravitation.
Now, then, we will ask first, "What is electricity?" And the simple answer must be, "We don't know." Well, but this need not necessarily be depressing. If the same question were asked about matter, or about energy, we should have likewise to reply, "No one knows."

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