In light of the fact that Church teaching makes a big deal of the notion of substance in its doctrine of Transubstantiation (how consecrating the Host changes its substance, but not its properties), I’m going to ramble on about the nature of substance for a little bit.
I remember once, some time after I had graduated from college a physics friend of my father said something like “I always thought Aristotle was a materialist, but a philosopher I know said he wasn’t; he was a substantialogist. What’s the difference?” At the time, I was unable to elucidate the difference, so I demurred, saying “I’m really not sure,” although I should have been able to do so. Anyway, I am now going to try to answer that question.
Materialists believe everything is made of matter. Substantialogists believe everything is made of substances (note the s). So what is the difference between matter and substance—that is, when you are using them in their strictest philosophical senses?
Well, matter has properties. Substance does not. Matter has mass, density, volume, shape. Those are all properties. You can visualize a glop of matter. You can feel matter. You can punch matter. Surprisingly, none of those things apply to substance. I think the best definition of substance is: that in which properties adhere. So, if you have a substance (by the technical philosophical definition, not the common sense definition where it means basically a physical chunk of something) you can’t ascribe a mass to it in itself, because mass is a property. You can’t ascribe a volume to it for the same reason. It is basically impossible to visualize substance. And philosophers recognize this.
So, you might ask, if you can’t visualize it, why do you even think it’s there? Well, it started with Aristotle, I believe. He said that you can’t see it but you apprehend its necessity with your intellect. With respect to the Church, Church leaders argue that when the Host is consecrated the substance of the small piece of unleavened bread changes, but its properties do not. It still looks exactly the same, but an undetectable change has happened: the substance of unleavened bread has transformed into another substance: a piece of the spiritual body of Jesus Christ, the Messiah.
Anyway, I hope that clears things up. Please recognize that I haven’t studied philosophy in about twenty five years or so; so, my explanation may be a little lacking.