I was reading some articles and discussions on the topic and got confused when I read that conventionally photons and all other massless particles aren't considered a form of matter. The Wiki article on matter begins with the statement that the term itself is poorly defined. I was thinking that a good way to define the term could be to say that matter is something which has mass or energy. This would also capture our intuitive understanding of the concept of material substance, since when dualists speak of the "non-material world", they certainly don't have photons in mind. If energy can be converted to matter and matter to energy, and only the sum is concerved, why aren't massless particles considered material?