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. But the fallacy and absurdity of
this argument will sufficiently appear from a slight examination
of what Mr Condorcet calls the organic perfectibility, or
degeneration, of the race of plants and animals, which he says
may be regarded as one of the general laws of nature.

I am told that it is a maxim among the improvers of cattle