The theory would have little value or significance, if the prospect of progress in the future depended on chance or the unpredictable discretion of an external will.
Fontenelle asserts implicitly the certainty of progress when he declares that the discoveries and improvements of the modern age would have been made by the ancients if they exchanged places with the moderns; for this amounts to saying that science will progress and knowledge increase independently of particular individuals.

If Descartes had not been born, some one else would have done his work; and there could have been no Descartes before the seventeenth century.

For, as he says in a later work, [Footnote: Preface des elemens de la geometrie de l'infini (OEuvres, x.p.40, ed.

1790).] "there is an order which regulates our progress.

Every science develops after a certain number of preceding sciences have developed, and only then; it has to await its turn to burst its shell." Fontenelle, then, was the first to formulate the idea of the progress, of knowledge, as a complete doctrine.