Betwixt unity and number there can be no medium; no more than betwixt existence and nonexistence.

After one object is supposed to exist, we must either suppose another also to exist; in which case we have the idea of number: Or we must suppose it not to exist; in which case the first object remains at unity.
To remove this difficulty, let us have recourse to the idea of time or duration.

I have already observd [Part II, Sect.

5.], that time, in a strict sense, implies succession, and that when we apply its idea to any unchangeable object, it is only by a fiction of the imagination, by which the unchangeable object is supposd to participate of the changes of the co-existent objects, and in particular of that of our perceptions.

This fiction of the imagination almost universally takes place; and it is by means of it, that a single object, placd before us, and surveyd for any time without our discovering in it any interruption or variation, is able to give us a notion of identity.