The ego judges a given perception as not coming from the outer world (i.e., as being of intrapsychic origin) by the operation of its positive capacity forcefully to oppose the cathexis of such a perception by countercathexis. whatever cannot in some way be dominated by countercathexis is considered by the ego to originate in external reality.

In other words, if a special dynamic equilibrium exists between the libidinous cathexis which brings about a perception, and the countercathexis from the ego which does not allow this perception to pass freely (countercathexis dominating), the ego regards what is perceived as intrapsychic, and vice versa.