  Whereas Locke argues that the human being might, for all we know, be matter with thought superadded, Norris claims in no uncertain terms that the human being is a substantial union. However, Norris can only argue for his claim within his ideal philosophy, which Locke rejects. But the grounds for rejection spring from the very view of ‘idea’ Locke espouses; an idea is generated and formed by sensation and reflection. This debate concerns, fundamentally, the ontological status of ideas, and how far ideas can guarantee a correspondence to reality external to thought. In this debate, the reader can fully appreciate one main difference between Cartesian rationalism and Lockean empiricism in late 17th Century England. For Locke, propositions about ideas are themselves mere mental propositions. As such, their certainty does not guarantee truth. For Norris, clear and distinct mental propositions reflect ideal propositions. Certainty about mental propositions does guarantee truth.