We are here dealing with the central problem of things, and we must not leave it till we have made it as plain as possible.

I will therefore re-state it in terms of another metaphor.

Let us compare the universal matter, with its infinity of molecules, to a number of balls on a billiard-table, set in motion by the violent stroke of a cue.

The balls at once begin to strike each other and rebound from the cushions at all angles and in all directions, and assume with regard to each other positions of every kind.

At last six of them collide or cannon in a particular corner of the table, and thus group themselves so as to form a human brain; and their various changes thereafter, so long as the brain remains a brain, represent the various changes attendant on a man's conscious life.