of motor elements.  A motor impulse or idea does not always result in apparent peripheral movement.  In the suppressed speech, which is the common language of thought, the possibility of incipient or incomplete motor innervations is well recognized.  But where the peripheral movement actually occurs it must be accounted for.  And as the cause here must be central, it seems reasonable to impute it to certain motor innervations which condition the shifting of the mental attitude and may be incipient merely, but which, if completed, result in the shifting of the eyes and the changes of bodily attitude which accompany the scrutiny of an external object.  And the sensory process is, to some extent at least, conditioned by the motor, if, indeed, the two are anything more than different aspects of one and the same process.[7]

   [7] Cf.  Muensterberg, H.:  ‘Grundzuege d.  Psychologie,’ Bd.  I.,
   Leipzig, 1900, S. 532.

But where, now, the subject is occupied in mentally tracing the boundaries of one of his two images he must inhibit all motor innervations incompatible with the innervations which condition such tracing:  the rival process must cease, and the rival image will fade.  He may, it is true, include both images in the same mental sweep.  The boundary line is not the only possible line of movement.  In fact, we may regard this more comprehensive glance as equivalent to an enlargement of the boundaries so as to include different mental objects, instead of different parts of but one.  Or, since the delimitation of our ‘objects’ varies with our attitude or aim, we may call it an enlargement of the object.  But in any case the mental tracing of a particular boundary or particular spatial dimensions seems to condition the sense of the corresponding content, and through inhibition of inconsistent movements to inhibit the sense of a different content.  No measure of the span of consciousness can, of course, be found in these reports.  The movements of the attention are subtle and swift, and there was nothing in the form of the experiments to determine at any precise instant its actual scope.  All we need assume, therefore, when the images are said to be seen together, is that neither has, for the time being, any advantage over the other in drawing attention to itself.  If in the complete observation, however, any such advantage appears, we may treat it as a case of inhibition.  By definition, an idea which assumes a place in consciousness which but for itself, as experiment indicates, another might occupy, inhibits the other.

[Illustration:  FIG. 3.]

TABLE III.