a statement about Julius Caesar, it is plain that Julius Caesar himself is not before our minds, since we are not acquainted with him. We have in mind some description of Julius Caesar: 'the man who was assassinated on the Ides of March', 'the founder of the Roman Empire', or, merely 'the man whose name was Julius Caesar'. (In this last description, Julius Caesar is a noise or shape with which we are acquainted.) Thus our statement does not mean quite what it seems to mean, but means something involving, instead of Julius Caesar, some description of him which is composed wholly of particulars and universals with which we are acquainted. The chief importance of knowledge by description is that it enables us to pass beyond the limits of our experience. In spite of the fact that we can only know truths which are wholly composed of terms which we have experienced in acquaintance, we can yet have knowledge by description of things which we have never experienced. In view of the very narrow range of our immediate experience, this result is vital, and until it is understood, much of our knowledge must remain mysterious and therefore doubtful. ALL IMPORTANT ATTEMPTS at establishing a theory of knowledge grow out of the problem concerning the certainty of human knowledge. And this problem in turn originates in the wish for absolute certainty. The insight that the statements of daily life and science can at best be only probable, that even the most general results of science, which all experiences confirm, can have only the character of hypotheses, has again and again stimulated philosophers since Descartes, and indeed, though less obviously, since ancient times, to search for an unshakeable, indubitable, foundation, a firm basis on which the uncertain structure of our knowledge could rest. The uncertainty of the structure was generally attributed to the fact that it was impossible, perhaps in principle, to construct a firmer one by the power of human thought. But this did not inhibit the search for the bedrock, which exists prior to all construction and does not itself vacillate. This search is a praiseworthy, healthy effort, and it is prevalent even among "relativists" and "sceptics, who would rather not acknowledge it." It appears in different forms and leads to odd differences of opinion. The problem of "protocol statements," their structure and function, is the latest form in which the philosophy or rather the decisive empiricism of our day clothes the problem of the ultimate ground of knowledge. What was originally meant by "protocol statements," as the name indicates, are those statements which