In order to understand the link between observation and science, Quine's naturalized epistemology must be able to identify and describe the process by which scientific knowledge is acquired. One form of this investigation is reliabilism which requires that a belief be the product of some reliable method if it is to be considered knowledge. Since naturalized epistemology relies on empirical evidence, all epistemic facts which comprise this reliable method must be reducible to natural facts. That is, all facts related to the process of understanding must be expressible in terms of natural facts. If this is not true, i.e. there are facts which cannot be expressed as natural facts, science would have no means of investigating them. In this vein, Roderick Chisholm argues that there are epistemic principles (or facts) which are necessary to knowledge acquisition, but may not be, themselves, natural facts. If Chisholm is correct, naturalized epistemology would be unable to account for these epistemic principles and, as a result, would be unable to wholly describe the process by which knowledge is obtained.