In responding to the problem that God is able to do all things yet it is unable to lie or deceive, Anselm says that the word “able” or “ability” can be used to describe not only real powers, but also deficiencies and weaknesses. If asked to list off all the things a screwdriver can do, we’d say things like “tighten and loosen screws” or “pry off paint-can lids” or “sabotage locks”, but we wouldn’t say “it can rust” or “it can have it’s tip deformed”. Corruptions and weaknesses aren’t listed among a things abilities, even though the ability to be any of these things is intrinsic and inseparable from them. Once we start speaking about corruptions or weaknesses as abilities of a thing, we’ve noticeably shifted the sense of ability such that it is wrong to make a single list that included both senses.
But the sense of what it means to corrupt needs some refinement. Explosives and fuels, for example, have a being inseparable from their own destruction. Dynamite that must remain forever a foot-long red stick or gasoline that could never corrupt into water vapor and carbon dioxide could never exist, except by accident. The first thing we would include among the abilities of such things involves their ceasing to be what they are when they give rise to something else. The same account would apply to fuels in nature, like food or nutrients. And so we hit the paradox that there are things whose proper being (that is, the expression of their abilities) consists in a moment when they cease to be at all. The proper work of some things takes place in the moment when, in fact, there is no longer the very thing supposed to be working. This leads us to divide what we mean by a being (that is, some subject that does certain things) into those things that have being of themselves, and those that have being only by another. In the measure that anything is a fuel, for example, its whole being is from another. Such things could not be if they could not be.
But it seems that the place we call nature is one where, even if any one thing is what it is, it is also a fuel source. There is certainly some reality about me that a starving lion or cannibal tribe chases after; and even the most primitive stuff of the universe (like a floating hydrogen cloud) is a fuel source. Aristotelians would take this as an occasion to notice that there is a division in being in any natural thing into matter and form. This is fine, but the point of the discourse here is to reveal matter and form in a particular way. Matter is not some co-equal principle with form that belongs to the proper being of the thing, as though it could be placed alongside form in this way. Matter or potency as such are outside of the proper being of a thing. “Matter” or “potency” in this sense are the measure in which a thing does not exist of itself. This is why the act of potential so far as it is potential is not being, but becoming or motion. Matter only enters into the being of something in the measure that it can be made formal, though there is an obvious contradiction in making matter be completely formal (though the closest thing to it might be the Medieval idea of the celestial bodies, where some form was thought to completely exhaust the possibility of some matter; and this category of being might at some point become important again to natural science.)
Following this line of inquiry about matter and form, the immaterial is simply that whose proper being exhausts what it is. An immaterial thing is just itself, as opposed to being a fuel source. The immaterial is much more solid and simple than the natural, being neither capable of being broken down for another or of admitting a proper account of itself as something other than what it is. In fact, a seemingly tautological statement (like a spade is a spade) seems to involve a sort of vision of reality that is only suggested by natural, physical being but can never be achieved by it. The ontological simplicity we want to attribute to chickens or oxygen or the various things we find around us cannot be verified by these things themselves in a complete way, and in this sense the world more suggest being than it gives us examples of it.