Particle

"... Where was the particle just before I made the measurement? There are three plausible answers to this question, and they serve to characterize the main schools of thought regarding quantum indeterminacy: ... realist ... orthodox ... agnostic ... Until fairly recently, all three positions (realist, orthodox, and agnostic) had their partisans. But in 1964 John Bell astonished the physics community by showing that it makes an observable difference whether the particle had a precise (though unknown) position prior to the measurement, or not. Bell's discovery effectively eliminated agnosticism as a viable option ..."

"David J. Griffiths, Introduction to Quantum Mechanics (2nd ed., 2016), pp. 3–4"

"One of the fundamental concepts of mechanics is that of a particle. By this we mean a body whose dimensions may be neglected in describing its motion. The possibility of so doing depends, of course, on the conditions of the problem concerned. For example, the planets may be regarded as particles in considering their motion about the Sun, but not in considering their rotation about their axes."

"There were two things that especially attracted me to the ideas of renormalization and quantum field theory. One of them was that the requirement that a physical theory be renormalizable is a precise and rational criterion of simplicity. In a sense, this requirement had been used long before the advent of renormalization theory. When Dirac wrote down the Dirac equation in 1928 he could have added an extra ‘Pauli’ term ... which would have given the electron an arbitrary anomalous magnetic moment. Dirac could (and perhaps did) say ‘I won’t add this term because it’s ugly and complicated and there’s no need for it.’ I think that in physics this approach generally makes good strategies but bad rationales. It’s often a good strategy to study simple theories before you study complicated theories because it’s easier to see how they work, but the purpose of physics is to find out why nature is the way it is, and simplicity by itself is I think never the answer. But renormalizability was a condition of simplicity which was being imposed for what seemed after Dyson’s 1949 papers ... like a rational reason, and it explained not only why the electron has the magnetic moment it has, but also (together with gauge symmetries) all the detailed features of the standard model of weak, electromagnetic, and strong, interactions, aside from some numerical parameters. The other thing I liked about quantum field theory during this period of tremendous optimism was that it offered a clear answer to the ancient question of what we mean by an elementary particle: it is simply a particle whose field appears in the Lagrangian. It doesn’t matter if it’s stable, unstable, heavy, light — if its field appears in the Lagrangian then it’s elementary, otherwise it’s composite."