Physicalism claims that the ontology of the world coincides with the ontology of physics. My paper offers a discussion and assessment of this claim. First, I explain why physicalism implies a straightforward ontological reductionism; hence, on an ontological reading, ‘non-reductive physicalism’ is a contradiction in terms. Second, I argue that, without further specification, the content of ‘the ontology of physics’ is completely undefined. Since this content is usually taken for granted in the philosophy of psychology and the philosophy of mind, the notion of physicalism has no positively determinable meaning. Third, by taking into account the full particulars of how observations of physical phenomena are realized in practice, I question the claim that reductive physicalism is confirmed by the cognitive achievements of connectionist networks. © 2001, Sage Publications. All rights reserved.