One challenge to the traditional concept of matter as tangible "stuff" came with the rise of field physics in the 19th century. Relativity shows that matter and <answer>energy</answer> (including the spatially distributed <answer>energy</answer> of fields) are interchangeable. This enables the ontological view that <answer>energy</answer> is prima materia and matter is one of its forms. On the other hand, the Standard Model of Particle physics uses quantum field theory to describe all interactions. On this view it could be said that fields are prima materia and the <answer>energy</answer> is a property of the field.
Ontological theory suggests that what is the main substance?
energy