What is "method of indivisibles"

Indivisible \In`di*vis"i*ble\, n.

1. That which is indivisible.

   By atom, nobody will imagine we intend to express a perfect indivisible, but only the least sort of natural bodies.
   --Digby.

2. (Geom.) An infinitely small quantity which is assumed to admit of no further division.

   Method of indivisibles, a kind of calculus, formerly in use, in which lines were considered as made up of an infinite number of points; surfaces, as made up of an infinite number of lines; and volumes, as made up of an infinite number of surfaces.

n. (context obsolete English) An old kind of calculus in which lines were considered as made up of an infinite number of points; surfaces, as made up of an infinite number of lines; and volumes, as made up of an infinite number of surfaces.