Thus this secular man, who has lived since the beginning of the world, has had his infancy in which he was absorbed by the most urgent needs of life; his youth in which he succeeded pretty well in things of imagination like poetry and eloquence, and even began to reason, but with more courage than solidity.

He is now in the age of manhood, is more enlightened, and reasons better; but he would have advanced further if the passion for war had not distracted him and given him a distaste for the sciences to which he has at last returned.
Figures, if they are pressed, are dangerous; they suggest unwarrantable conclusions.

It may be illuminative to liken the development of humanity to the growth of an individual; but to infer that the human race is now in its old age, merely on the strength of the comparison, is obviously unjustifiable.

That is what Bacon and the others had done.

The fallacy was pointed out by Fontenelle.
From his point of view, an "old age" of humanity, which if it meant anything meant decay as well as the wisdom of experience, was contrary to the principle of the permanence of natural forces.