Prefaces were written to give the sketch and purpose of chapters not yet composed.

The _Novum Organum_ had been written and rewritten twelve times over.

Bacon kept his papers, and we can trace in the unused portion of those left behind him much of the progress of his work, and the shapes which much of it went through.

The _Advancement_ itself is the filling-out and perfecting of what is found in germ, meagre and rudimentary, in a _Discourse in Praise of Knowledge_, written in the days of Elizabeth, and in some Latin chapters of an early date, the _Cogitationes de Scientia Humana_, on the limits and use of knowledge, and on the relation of natural history to natural philosophy.

These early essays, with much of the same characteristic illustration, and many of the favourite images and maxims and texts and phrases, which continue to appear in his writings to the end, contain the thoughts of a man long accustomed to meditate and to see his way on the new aspects of knowledge opening upon him.