Only the assumption such is made by Kant and is common to the traditional theories that all experience is inherently cognitive leads to the doctrine that perception of the patient is acase of knowledge. In reality the original perception furnishes the problem for knowing j it is something to be known, not an object of knowing. And in knowing, the first thing to be done is to select from the mass of presented qualities those which, in distinction from other qualities, throw light upon the nature of the trouble. As they are deliberately selected, being discriminated by special tech- nical operations, they become data; these are called sensible simply because of the role of sense organs in their generation. They may then be formulated as the subject-matter of primi- tive existential propositions. But even so, there is no class of such propositions In general. Each inquiry yields its own primitive existential propositions, even though they all agree in having for their objects qualities which investigation reveals to be connected with the use of organs of sense. Moreover, these primitive propositions are such only in alogical sense as distinct from being empirically primitive, and they are only hypothetical or conditional. This statement does not imply that their existence is hypothetical jperception, as far as it is properly conducted, warrants their existence. But their status in inquiry is tentative. Many, perhaps most, errors in physical inference arise from taking as data things that are not data for the problem in hand; they undoubtedly exist, but they are not the evidence that is demanded.