Probably the most distinctive feature of any instinctive act is the fact that it is invariably performed with interest or excitement. Often, as we have seen, this excitement is so pronounced as to be definable as emotion, passion: at other times it is more vaguely felt, not obviously coloured by sensation. It is never indifferent. Moreover, there is in activity of this kind a feeling of satisfaction which so flavours the action it accompanies with pleasurable spontaneity as to render it peculiarly effortless. The interest felt is dynamic because it is the eager response of the whole organism, glad in its own self-awareness, exultant to feel that it is being used for its natural purpose,—not merely a mental or physical reaction.