This book highlights the historical roots and important philosophical debates that accompanied the introduction of different theoretical entities. Understanding what we see through objects that we cannot perceive may seem a paradox. Yet it is precisely such elusive objects-to the point that they cannot be revealed, like the quarks-which make it possible to situate what we call matter in the sphere of the intelligible. The search for a fundamental unit or units to which the construction of the polychromatic world around us can be traced has marked the history of thought, a search that has pushed inquiry into smaller and smaller regions of our universe. The result has been disruptive: we have disengaged from the safe references suggested by the most immediate intuition, lost the reassuring comfort of simple images, and abandoned the support of established philosophical categories. Terms such as particle, vacuum, and others have no right of citizenship in contemporary physics in their common meaning. Particles are not small bodies, and vacuum is not the absence of matter. Such terms, like many others, have become theoretical entities because they are increasingly unreachable by sensible experience. We cannot answer the legitimate question "what are they?". What we can do instead-and what is done in this book-is to follow the evolution of their meanings through a journey among invisible objects and among images that have gradually become less and less imaginable. It is a singular journey in which we will meet the astonishing properties of the quantum vacuum, its seething virtual particles, the astonishing idea, using Dyson&s words, "that our solid world of trees and stones may be made of quantum fields and nothing else." Most importantly, we will encounter powerful decoders that enable the scientific community to make theoretical entities "visible" by means of shared representations.