The double motion is a kind of a double tie, and binds the objects together in the closest and most intimate manner.
The second marriage of a mother breaks not the relation of child and parent; and that relation suffices to convey my imagination from myself to her with the greatest ease and facility.

But after the imagination is arrived at this point of view, it finds its object to be surrounded with so many other relations, which challenge its regard, that it knows not which to prefer, and is at a loss what new object to pitch upon.

The ties of interest and duty bind her to another family, and prevent that return of the fancy from her to myself, which is necessary to support the union.

The thought has no longer the vibration, requisite to set it perfectly at ease, and indulge its inclination to change.

It goes with facility, but returns with difficulty; and by that interruption finds the relation much weakened from what it would be were the passage open and easy on both sides.
Now to give a reason, why this effect follows not in the same degree upon the second marriage of a father: we may reflect on what has been proved already, that though the imagination goes easily from the view of a lesser object to that of a greater, yet it returns not with the same facility from the greater to the less.