The grand and consistent theory of Newton will be placed upon the same footing as the wild and eccentric hypotheses of Descartes.

In short, if the laws of nature are thus fickle and inconstant, if it can be affirmed and be believed that they will change, when for ages and ages they have appeared immutable, the human mind will no longer have any incitements to inquiry, but must remain fixed in inactive torpor, or amuse itself only in bewildering dreams and extravagant fancies.
The constancy of the laws of nature and of effects and causes is the foundation of all human knowledge, though far be it from me to say that the same power which framed and executes the laws of nature may not change them all 'in a moment, in the twinkling of an eye.' Such a change may undoubtedly happen.

All that I mean to say is that it is impossible to infer it from reasoning.

If without any previous observable symptoms or indications of a change, we can infer that a change will take place, we may as well make any assertion whatever and think it as unreasonable to be contradicted in affirming that the moon will come in contact with the earth tomorrow, as in saying that the sun will rise at its usual time.
With regard to the duration of human life, there does not appear to have existed from the earliest ages of the world to the present moment the smallest permanent symptom or indication of increasing prolongation.

The observable effects of climate, habit, diet, and other causes, on length of life have furnished the pretext for asserting its indefinite extension; and the sandy foundation on which the argument rests is that because the limit of human life is undefined; because you cannot mark its precise term, and say so far exactly shall it go and no further; that therefore its extent may increase for ever, and be properly termed indefinite or unlimited.