Recently the attempt has been made to demonstrate Heidegger's relevance to the concerns of analytic philosophers. A focus for this effort has been the criticism in his early work of Cartesian ontology. While a number of important works have mapped out this area of Heidegger's thought, a crucial task has not been carried out, namely that of assessing how Heidegger can accommodate those phenomena which motivate the Cartesian to adopt his highly counter-intuitive ontology. As long as we fail to examine how Heidegger's early ontology copes with the possibilities of error and of hallucination, the suspicion will remain that Heidegger is simply insensitive to those phenomena on which the Cartesian focuses. Neither Heidegger nor the Cartesian have been done any favours by commentators showing little inclination to bring the opponents into closer combat. This paper attempts to correct that omission.