Sometimes the mind fixes itself with so much earnestness on the contemplation of some objects, that it turns their ideas on all sides; marks their relations and circumstances; and views every part so nicely and with such intention, that it shuts out all other thoughts, and takes no notice of the ordinary impressions made then on the senses, which at another season would produce very sensible perceptions: at other times it barely observes the train of ideas that succeed in the understanding, without directing and pursuing any of them: and at other times it lets them pass almost quite unregarded, as faint shadows that make no impression.