confidently predict that D will follow. But, however often
we have observed such a sequence, and however many similar sequences we
may have observed, we are no nearer to knowing _why_ D should follow ABC:
we can only know that it always does: and on the strength of that
knowledge we infer, with a probability which we do no doubt for practical
purposes treat as a certainty, that it always will. But on reflection we
can see no reason why a wave of ether of a certain length should produce
red rather than blue, a colour rather than a sound. There, as always, we
discover nothing but succession, not necessary connexion.

These cases of unvaried succession among phenomena, it should be
observed, are quite different from cases of real necessary connexion. We
don't want to examine thousands of instances of two {35} added to two to
be quite sure that they always make four, nor in making the inference do
we appeal to any more general law of Uniformity. We simply see that it
is and always must be so. Mill no doubt tells us he has no difficulty in
supposing that in the region of the fixed stars two and two might make
five, but nobody believes him. At all events few of us can pretend to
such feats of intellectual elasticity. No amount of contradictory
testimony from travellers to the fixed stars, no matter whether they were
Bishops of the highest character or trained as Professors of physical
Science, would induce us to give a moment's credence to such a story. We
simply see that two and two must make four, and that it is inconceivable
they should ever, however exceptionally, make five. It is quite
otherwise with any case of succession among external phenomena, no matter
how unvaried. So long as we confine ourselves to merely physical
phenomena (I put aside for the moment the case of conscious or other
living beings) nowhere can we discover anything but succession; nowhere
do we discover Causality in the sense of a necessary connexion the
reversal of which is inconceivable.

Are we then to conclude that there is no such thing as Causality, that in
searching for a cause of everything that happens, we are pursuing a mere
will o' the wisp, using a mere _vox nihili_ which has {36} as little
meaning for the reflecting mind as fate or fortune? Surely, in the very
act of making the distinction between succession and causality, in the
very act of denying that we can discover any causal connexion between one
physical phenomenon a