What's the matter?

"When a stranger, hearing that I am a physicist, asks me in what area of physics I work, I generally reply that I work on the theory of elementary particles. Giving this answer always makes me nervous. Suppose that the stranger should ask, “What is an elementary particle?” I would have to admit that no one really knows." Steven Weinberg in What is an Elementary Particle?

"Just what are electrons, quarks and so on, though? The best we can do at this stage is simply to refer to the mathematical equations that they satisfy..." Roger Penrose in What is Reality?

"So, what is an electron? An electron is a particle and a wave; it is ideally simple and unimaginably complex; it is precisely understood and utterly mysterious... No single answer does justice to reality." Frank Wilczek in The Enigmatic Electron