From the improvement of medicine, from the use of more wholesome food and habitations, from a manner of living which will improve the strength of the body by exercise without impairing it by excess, from the destruction of the two great causes of the degradation of man, misery, and too great riches, from the gradual removal of transmissible and contagious disorders by the improvement of physical knowledge, rendered more efficacious by the progress of reason and of social order, he infers that though man will not absolutely become immortal, yet that the duration between his birth and natural death will increase without ceasing, will have no assignable term, and may properly be expressed by the word ‘indefinite’.  He then defines this word to mean either a constant approach to an unlimited extent, without ever reaching it, or an increase.  In the immensity of ages to an extent greater than any assignable quantity.

But surely the application of this term in either of these senses to the duration of human life is in the highest degree unphilosophical and totally unwarranted by any appearances in the laws of nature.  Variations from different causes are essentially distinct from a regular and unretrograde increase.  The average duration of human life will to a certain degree vary from healthy or unhealthy climates, from wholesome or unwholesome food, from virtuous or vicious manners, and other causes, but it may be fairly doubted whether there is really the smallest perceptible advance in the natural duration of human life since first we have had any authentic history of man.  The prejudices of all ages have indeed been directly contrary to this supposition, and though I would not lay much stress upon these prejudices, they will in some measure tend to prove that there has been no marked advance in an opposite direction.

It may perhaps be said that the world is yet so young, so completely in its infancy, that it ought not to be expected that any difference should appear so soon.

If this be the case, there is at once an end of all human science.  The whole train of reasonings from effects to causes will be destroyed.  We may shut our eyes to the book of nature, as it will no longer be of any use to read it.  The wildest and most improbable conjectures may be advanced with as much certainty as the most just and sublime theories, founded on careful and reiterated experiments.  We may return again to the old mode of philosophising and make facts bend to systems, instead of establishing systems upon facts.  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.