While, to speak figuratively, the minute anatomy of consciousness is thus difficult with respect to longitudinal sections of the mental column, it is no less difficult with respect to transverse sections. Under ordinary circumstances, external impressions persist so that they can be transfixed by a deliberate act of attention, and objects rarely flit over the external scene so rapidly as to allow us no time for a careful recognition of the impression. Not so in the case of the internal region of mind. The composite states of consciousness just described never remain perfectly uniform for the shortest conceivable duration. They change continually, just as the contents of the kaleidoscope vary with every shake of the instrument. Thus, one shade of feeling runs into another in such a way that it is often impossible to detect its exact quality; and even when the character of the feeling does not change, its intensity is undergoing alterations so that an accurate observation of its quantity is impracticable. Also, in this unstable shifting internal scene features may appear for a duration too short to allow of close recognition. In this way it happens that we cannot sharply divide the feeling of the moment from its antecedents and its consequents. If, now, we take these facts in connection with what has been said above respecting the nature of the process of introspection, the probability of error will be made sufficiently clear. To transfix any particular feeling of the moment, to selectively attend to it, and to bring it under the proper representation, is an operation that requires time, a time which, though short, is longer than the fugitive character of so much of our internal mental life allows. From all of which it would appear to follow that it must be very easy to overlook, confuse, and transform, both as to quality and as to quantity, the actual ingredients of our internal consciousness.