Those require a continued existence, or otherwise lose, in a great measure, the regularity of their operation.

I am here seated in my chamber with my face to the fire; and all the objects, that strike my senses, are contained in a few yards around me.

My memory, indeed, informs me of the existence of many objects; but then this information extends not beyond their past existence, nor do either my senses or memory give any testimony to the continuance of their being.
When therefore I am thus seated, and revolve over these thoughts, I hear on a sudden a noise as of a door turning upon its hinges; and a little after see a porter, who advances towards me.

This gives occasion to many new reflections and reasonings.

First, I never have observed, that this noise coued proceed from any thing but the motion of a door; and therefore conclude, that the present phaenomenon is a contradiction to all past experience, unless the door, which I remember on the other side the chamber, be still in being.