 Since various arguments seem to speak against a particle interpretation, the allegedly only alternative, namely a field interpretation, is often taken to be the appropriate ontology of QFT. So let us see what a physical field is and why QFT may be interpreted in this sense. A classical point particle can be described by its position \(\mathbf{x}(t)\) and its momentum \(\mathbf{p}(t)\), which change as the time \(t\) progresses. So there are six degrees of freedom for the motion of a point particle corresponding to the three coordinates of the particle’s position and three more coordinates for its momentum. In the case of a classical field one has an independent value for each single point \(\mathbf{x}\) in space, where this specification changes as time progresses. The field value \(\phi\) can be a scalar quantity, like temperature, a vectorial one as for the electromagnetic field, or a tensor, such as the stress tensor for a crystal. A field is therefore specified by a time-dependent mapping from each point of space to a field value \(\phi(\mathbf{x},t)\). Thus a field is a system with an infinite number of degrees of freedom, which may be restrained by some field equations. Whereas the intuitive notion of a field is that it is something transient and fundamentally different from matter, it can be shown that it is possible to ascribe energy and momentum to a pure field even in the absence of matter. This somewhat surprising fact shows how gradual the distinction between fields and matter can be.