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Let us turn to the communications between the Ucs. and the other systems, less in order to establish anything new than in order to avoid omitting what is most prominent. At the roots of instinctual activity the systems communicate with one another most extensively. One portion of the processes which are there excited passes through the Ucs., as through a preparatory stage, and reaches the highest psychical development in the Cs.; another portion is retained as Ucs. But the Ucs. is also affected by experiences originating from external perception. Normally all the paths from perception to the Ucs. remain open, and only those leading on from the Ucs. are subject to blocking by repression.

It is a very remarkable thing that the Ucs. of one human being can react upon that of another, without passing through the Cs. This deserves closer investigation, especially with a view to finding out whether preconscious activity can be excluded as playing a part in it; but, descriptively speaking, the fact is incontestable.

The content of the system Pcs. (or Cs.) is derived partly from instinctual life (through the medium of the Ucs.), and partly from perception. It is doubtful how far the processes of this system can exert a direct influence on the Ucs.; examination of pathological cases often reveals an almost incredible independence and lack of susceptibility to influence on the part of the Ucs. A complete divergence of their trends, a total severance of the two systems, is what above all characterizes a condition of illness. Nevertheless, psycho-analytic treatment is based upon an influencing of the Ucs. from the direction of the Cs., and at any rate shows that this, though a laborious task, is not impossible. The derivatives of the Ucs. which act as intermediaries between the two systems open the way, as we have already said, towards accomplishing this. But we may safely assume that a spontaneously effected alteration in the Ucs. from the direction of the Cs. is a difficult and slow process.