"But in so far as we apply perception and judgment in equal measure, it may easily happen that a personality appears to us as both introverted and extraverted, so that we cannot at once decide to which attitude the superior function belongs. In such cases only a thorough analysis of the function qualities can help us to a sound opinion. During the analysis we must observe which function is placed under the control and motivation of consciousness, and which functions have an accidental and spontaneous character. The former is always more highly differentiated than the latter, which also possess many infantile and primitive qualities."
(Psychological Types, 427-8.)