This matter of meaningless questions is a very subtle thing which may poison much more of our thought than that dealing with purely physical phenomena. I believe that many of the questions asked about social and philosophical subjects will be found to be meaningless when examined from the point of view of operations. It would doubtless conduce greatly to clarity of thought if the operational mode of thinking were adopted in all fields of inquiry as well as in the physical. Just as in the physical domain, so in other domains, one is making a significant statement about his subject in stating that a certain question is meaningless. In order to emphasize this matter of meaningless questions, I give here a list of questions, with which the reader may amuse himself by finding whether they have meaning or not. (1) Was there ever a time when matter did not exist? (2) May time have a beginning or an end? (3) Why does time flow? (4) May space be bounded? (5) May space or time be discontinuous? (6) May space have a fourth dimension, not directly detectible, but given indirectly by inference? (7) Are there parts of nature forever beyond our detection? (8) Is the sensation which I call blue really the _same_ as that which my neighbor calls blue? Is it possible that a blue object may arouse in him the same sensation that a red object does in me and _vice versa_? (9) May there be missing integers in the series of natural numbers as we know them? (10) Is a universe possible in which 2 + 2 ≠ 4? (11) Why does negative electricity attract positive? (12) Why does nature obey laws? (13) Is a universe possible in which the laws are different? (14) If one part of our universe could be completely isolated from the rest, would it continue to obey the same laws? (15) Can we be sure that our logical processes are valid? GENERAL COMMENTS ON THE OPERATIONAL POINT OF VIEW To adopt the operational point of view involves much more than a mere restriction of the sense in which we understand "concept," but means a far-reaching change in all our habits of thought, in that we shall no longer permit ourselves to use as tools in our thinking concepts of which we cannot give an adequate account in terms of operations. In some respects thinking becomes simpler, because certain old generalizations and idealizations become incapable of use; for instance, many of the speculations of the early natural philosophers become simply unreadable. In other respects, however, thinking becomes much more difficult, because the operational implications of a concept are often very involved. For example, it is most difficult to grasp adequately all that is contained in the apparently simple concept of "time," and requires the continual correction of mental tendencies which we have long unquestioningly accepted.