D. M. Armstrong is one of the leading philosophers of the past 50 years. He is an Australian mostly known for his writings on metaphysics. This is a short, well-written, relatively easy to understand summary of his metaphysical system.

Armstrong is a materialist or physicalist, who believes that nothing exists except space-time and its contents. He further holds that the world is made up of contingent states of affairs (or facts) and that states of affairs are made up of particulars and universals (e.g. protons and their mass). Laws of nature concern relations between types of states of affairs. 

Universals are only identifiable through empirical means. They only exist if they are exemplified or instantiated. This means that a given number exists if and only if there is a group of particulars that instantiates that number. The number 2 exists, for example, since Mars has 2 moons. Numbers that are not exemplified in this way are mere possibilia. On this basis, however, all but the very largest numbers exist, since any particulars may comprise a group, for example, the group consisting of my desk and the planet Mars instantiates the number 2 (this is called a “mereological sum”).

Armstrong discusses the existence of mind in his final chapter. He endorses an identity theory of mind — mental events and processes are identical to physical events and processes in the brain. He also endorses an identity theory of perception, but wasn’t quite clear to me what he meant by this. Armstrong admits that his theory of the mind faces three particular problems: the existence of consciousness (by which he means “our unmediated access to (some) of our own mental processes”), qualia (secondary qualities like color and taste) and intentionality (how thought and language are “about” something that might not exist). 

Armstrong is more troubled about qualia and intentionality than consciousness. I’m more troubled by qualia than consciousness or intentionality. If the redness of an apple isn’t actually part of the apple, and there’s nothing red inside our heads either, where is the redness at anyway? Armstrong considers his blue mouse pad: “Perception presents us with the blueness as an objective property of something in the world and I think we should accept this, accept that the blue colour is in the world qualifying the pad… I want to identify the colour surface with what the physicists tell us is going on there”. Maybe that’s true, but I’m not convinced.  (3/30/13)