Full Idea
Armstrong and Lewis said that mental items were defined in terms of typical causes and effects; if, as seems likely, research reveals that a particular causal niche is occupied by a physical state, it follows that pain is a physical state.
Gist of Idea
If pains are defined causally, and research shows that the causal role is physical, then pains are physical
Source
report of David M. Armstrong (A Materialist Theory of Mind (Rev) [1968]) by William Lycan - Introduction - Ontology p.5
Book Reference
'Mind and Cognition (2nd Edn)', ed/tr. Lycan,William [Blackwell 1999], p.5
A Reaction
I am not fully convinced of the first step in the argument. It sounds like the epistemology and the ontology have got muddled (as usual). We define mental states as we define electrons, in terms of observed behaviour, but what are they?