This line of thought led Ockham quite naturally to offer an account of the metaphysical status of general concepts. To what Aristotelian category do they belong? Are concepts in the human mind substances, that is, ‘things’ in their own right, and so are in their own way as real and independent as people and horses? Or are they instead accidents, that is, not stand-alone entities, but rather modifications of stand-alone entities, the way for Aristotle tall is not a stand-alone subject, but simply a way that a stand-alone subject can be? Ockham began his career defending the first option, but ended it defending the latter. The cause of this transformation was the sustained attacks of Walter Chatton.