[woopsn]

What can we see? Vapor trails, patterns burned into a plate, etc. The evidence of some "thing" slowing down, perhaps, but really the environment that slowed it down necessarily changing irreversibly. Irreversible effects cannot be arbitrarily small (apparently). We never see a particle, only effects such as these -- if we did see anything else, well, we couldn't possibly remember. Particles are an attempt to explain/theorize evidence that is fundamentally observational.