In his paper, MacKay reviews his Node Structure theory of error detection, but precedes it with a critical discussion of the Perceptual Loop theory of self-monitoring proposed in Levelt . The present commentary is concerned with this latter critique and shows that there are more than casual problems with MacKay's argumentation.

The Popperian epistemology underlying Levelt's commentary and other aspects of contemporary psychology has limited application and, in particular, does not apply to the creation or development of theory, the main goal of MacKay . This is relevant to Levelt's questions, “What has changed?” and “What is the harvest?”: From a non-Popperian perspective, both changes and harvest are greater than Levelt's commentary would suggest and carry implications for the field at large.