m 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.

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