But the theory of the progress of knowledge includes and acquires its value by including the indefinite future.

This step was taken by Fontenelle.

The idea had been almost excluded by Bacon's misleading metaphor of old age, which Fontenelle expressly rejects.

Man will have no old age; his intellect will never degenerate; and "the sound views of intellectual men in successive generations will continually add up." But progress must not only be conceived as extending indefinitely into the future; it must also be conceived as necessary and certain.

This is the second essential feature of the theory.